Lymphotrophic nanoparticle-enhanced magnetic resonance imaging for nodal clinical
target volume delineation in the radiotherapy treatment planning of pelvic malignancies:
Derivation of a class solution nodal clinical target volume
By
Robert Edward Dinniwell
A thesis submitted in conformity with the requirements
for the degree of M. Sc.
Graduate Department of the Institute of Medical Science
University of Toronto
© Copyright by Robert Edward Dinniwell (2010)
ii
Lymphotrophic nanoparticle-enhanced magnetic resonance imaging for nodal clinical
target volume delineation in the radiotherapy treatment planning of pelvic malignancies:
Derivation of a class solution nodal clinical target volume
Master of Science 2010
Robert Edward Dinniwell
Graduate Department of the Institute of Medical Science
University of Toronto
Dextran-coated ultra-small, superparamagnetic, iron oxide particles (USPIO) have been
proposed as magnetic resonance (MR) lymph node contrast agents. This thesis
analyzed the topographic distributions of the pelvic and inguinal lymph nodes and
quantified their spatial relations with the adjacent vascular system. We hypothesized
that USPIO would facilitate identification of normal lymph nodes in a manner superior to
that afforded by computed tomography or unenhanced MR, but using current clinically
available scanners would be unlikely to identify microscopic nodal metastases. We have
constructed a high quality nodal atlas describing probability distributions for lymph node
number, size and position. Using this model, we then defined a generic three-
dimensional nodal clinical target volume and a means of accurate delineation of this
volume in a three-dimensional representation. This is the most quantitative assessment
of the pelvic and inguinal lymphatics to date and will help to improve the successful
targeting of lymph nodes for radiotherapy.
iii
Acknowledgements
There are many people whom I want to thank for their patience, support and mentorship
during my MSc. studies. Dr. Michael Milosevic, my primary supervisor, whose focus and
insight were fundamental in helping to define this project. Dr. Masoom A. Haider, with
his imaging expertise and guidance, was fundamental to the success of our
collaboration. I would additionally like to thank Drs. Kartik Jhaveri, Michael Jewett,
David Jaffray and Anthony Fyles.
Ami Syed and Anna Kirilova whose help with the execution of the trial enabled it to run
smoothly. Dr. Gregory Czarnota who enabled us to obtain the largest series in
publication and the fastest accrual rate anywhere. Dr. Philip Chan whose assistance
and support has been invaluable in this as well as other endeavors.
This work was made possible with the financial assistance from an ACURA Canadian
Association of Radiation Oncology grant; the Canadian Prostate Cancer Research
Initiative; and the CIHR Excellence in Radiation Research for the 21st Century
(EIRR21). I would additionally like to thank Dr. Mukesh G. Harisinghani, Department of
Abdominal Imaging, Massachusetts General Hospital, Dr. Paula Jacobs, and AMAG
Pharmaceuticals Inc., Lexington, MA for their valued support during this study.
Lastly, I wish to thank my friends, family and wife, Julie, for her support and
understanding.
iv
Table of Contents
Abstract …………………………………………………………………………………….…….ii
Acknowledgements ……………………………..……………………………………………..iii
List of Tables .…………………………………..……………………………….………….......v
List of Figures …………………………………...…………………………….…………..…..vii
List of Abbreviations ……….…………………...…………………………...…………...…..xiii
Chapter 1. Background and Literature Review ..……………….…………....……………...1
Chapter 2. …………………….……………...………………………………………………..46
Pelvic Lymph Node Topography for Radiotherapy Treatment Planning from
Ferumoxtran-10 Contrast-Enhanced MR Imaging
Dinniwell R, Chan P, Czarnota G, Haider MA, Jhaveri K, Jewett M, Fyles A, Jaffray D,
Milosevic M. Pelvic lymph node topography for radiotherapy treatment planning from
ferumoxtran-10 contrast-enhanced magnetic resonance imaging. Int J Radiat Oncol Biol
Phys. 2009 Jul 1;74(3):844-851. Published as the cover feature.
Chapter 3. …………………….………………...………………………………….…………72
Inguinal Lymph Node Topography for Radiotherapy Treatment Planning from
Contrast-Enhanced MR Imaging
Chapter 4. General Discussion ..……………...……………………………….…………..99
Chapter 5. Conclusions and Future Directions ……………………………………….…117
References ………………….…..……………...………………………………..…………..125
v
List of Tables
Table 2.1: Pelvic lymph node frequency and distribution ………………………………...58
Table 2.2: Spatial distribution of lymph node tissue in relation to the closest vessel in
each vascular segment ….……………………………………………………………………63
Table 2.3: Spatial distribution of lymph node tissue in relation to the closest vessel in
each vascular segment and musculoskeletal land markers ……………………….……..64
Table 3.1: Vessel combinations and the spatial distribution of lymph node tissue in
relation to the closest vascular segment as determined by the VHM dataset ...............84
Table 3.2: Spatial distribution of lymph node tissue in relation to the closest vascular
segment (Femoral Artery and Vein) derived from the 30 patient series .......................88
Table 3.3: Spatial distribution of lymph node tissue in relation to the closest vascular
segment (Femoral Artery and Vein and Superficial Circumflex Iliac Vein) derived from
the 30 patient series ...................................................................................…………....89
Table 3.4: Anisotropic analysis of the spatial distribution of lymph node tissue in relation
to the closest vascular segment to the closest vascular segment derived from the 30
patient series ……............................…............…………………………………………....90
vi
Table 4.1: Comparison of recommendations for nodal CTV delineation guidelines ...109
vii
List of Figures
Figure 1.1: Diagrammatic representation of a lymph node in cross-section showing the
entry and passage of lymph, its blood supply and the distribution of macrophages within
its interior. ..................................................................................................………..…….5
Figure 1.2: Representation of lymph (blue and yellow) draining from a lymph territory
(blue) into a first echelon lymph node shown in cross-section. ......................................6
Figure 1.3: Schematic depiction of the interconnections between lymphatic capillaries,
collectors, vessels and nodes for adjacent lymphatic territories. ..........…………....…....7
Figure 1.4: Schematic of the routes of lymph drainage from the lymphatic territories and
the proportion of the territory specific lymph in the lymph nodes downstream. ......…….8
Figure 1.5: (Left) Engraved front plate of Vasorum lymphaticorum corporis humani
historia et ichnographia and (Right) apparatus used to infuse mercury into the cadaveric
specimens and depictions of the efferent and afferent lymphatics with associated lymph
nodes. ....………………………………………………………..………………….…….….…11
Figure 1.6: (Left) Representative engraved plates from Vasorum lymphaticorum corporis
humani historia et ichnographia depicting the inguinal, iliac and aorto-caval lymphatic
vessels and nodes within the pelvis and retroperitoneum and (Right) the left inguinal
region showing the exquisite detail inherent in this work. ...........................…………….12
viii
Figure 1.7: (a) High resolution pelvic axial section from the Visible Human Anatomic
Series illustrating (b) small lymph nodes (arrows) close to the left external iliac vessels.
(c) Three-dimensional computer rendering derived from this dataset showing the
relationship between 177 pelvic and para-aortic lymph nodes (green) and the adjacent
vessels (red and blue). ....……………………………….……………………………………14
Figure 1.8: Frontal projection of the distal lumbar spine and bony pelvis depicting the
major vascular segments of interest. …………………………..………..………………….17
Figure 1.9: Frontal oblique projection of the distal lumbar spine and bony pelvis
depicting the major vascular segments of interest. ..…………………………..……….....18
Figure 1.10: Frontal projection of the distal lumbar spine and bony pelvis depicting the
major vascular segments of interest within the femoral triangle. ...…………..……….....22
Figure 1.11: Diagrammatic representation of the distribution of the deep and superficial
inguinal lymph nodes within the femoral triangle. ...……………………………………….23
Figure 1.12: Diagrammatic representation of the distribution and routes of lymphatic
drainage for the inguinal and pelvic lymph nodes. .........................................................25
Figure 1.13: (A) diagrammatic representation of a normal lymph node in cross-section
following USPIO administration and the corresponding MR appearance of the lymph
node. (B) lymph node with a moderate cancer burden and its post-contrast MR
ix
appearance. (C) lymph node with a minimal cancer burden and its post-contrast MR
appearance. ...................................................................................................................34
Figure 1.14: (a) T2-w pre-contrast axial MR image of the pelvis at the level of the
acetabulum. Inset figure depicts a normal-size proximal inguinal lymph node with similar
signal characteristics to the surrounding tissues. (b) T2-w post-contrast MR image
showing uniform distribution of USPIO contrast agent as evidenced by the pattern of
signal loss evident in the inset image. ...........................................................................35
Figure 1.15 Depiction of radiotherapy target volumes: GTV - GrossTarget Volume, CTV
- Clinical Target Volume, PTV - Planning Target Volume, IM - Internal Margin, and SM -
Set-up Margin. ...............................................................................................................38
Figure 1.16 Conventional radiotherapy plan for a carcinoma of the anal canal (top) and
an intensity modulated plan (bottom) illustrating a steep dose gradient at the edge of the
high dose volume (green) and reduced dose to bladder (B) and genitalia (G).
........................................................................................................................................42
Figure 2.1 A to D: Cranial to caudal axial T2w FSE, T2* GRE and T1w images superior
(A) to inferior (C and D) the bifurcation of the abdominal aorta. (Red arrow denotes the
division of the aorta in the right and left common iliac arteries). …………………………52
x
Figure 2.2: A three-dimensional surface rendering of the contoured pelvic lymph nodes
identified using ferumoxtran-10 in relation to the pelvic vasculature for a typical patient.
Arteries are indicated in red, veins in blue and lymph nodes in green. …………………55
Figure 2.3: A three-dimensional surface rendering of the contoured pelvic lymph nodes
(a) and a schematic representation (b) of the method used to determine the spatial
distribution of lymphatic tissue. The distance from the center of each lymph node
volume unit (green) to the closest artery (red) and vein (blue) was calculated in three
dimensions. ...………………………………………………………………………………….56
Figure 2.4: Histograms for a typical patient illustrating the spatial distribution of nodal
tissue in relation to the closest edge of the nearest artery or vein in each anatomic nodal
region: a) distal para-aortic; b) common iliac; c) external iliac; d) internal iliac; e) pelvic
side-wall and f) the sub-aortic/presacral. Note the different Y-axis scales, reflecting
differences in total lymphatic nodal volume from one region to another. .………………61
Figure 2.5: Pelvic nodal CTV at six cranio-caudal levels in the pelvis developed using
our population-based model of lymph node topography. The vascular, sacral and pelvic
sidewall expansions were trimmed to exclude bone, muscle, fascia, bladder and bowel.
(a) Distal para-aortic, (b) Bifurcation of the aorta and inferior vena cava, (c) Inferior to
the bifurcation of the common iliac vessels, (d) Mid-pelvis, (e) Cranial to the
acetabulum, and (f) Mid-acetabulum. …………………………………….............……67-68
xi
Figure 3.1: (A) A three-dimensional image rendering of the contoured lymph nodes,
vasculature and bony anatomy derived from the VHM with a representative image
adjacent. (B) three-dimensional rendering of pelvic and inguinal vasculature (1 inguinal
ligament, 2 sartorius muscle, 3 adductor longus muscle, 4 femoral artery and vein, 5
superficial circumflex iliac vein, 6 great saphenous vein, 7 inferior epigastric vein and
the 8 external pudendal vein) and (C) the corresponding lymph nodes. .......................80
Figure 3.2: (A) A three-dimensional surface rendering of the contoured left inguinal
lymph nodes with a schematic representation of the method used to determine the
spatial distribution of lymphatic tissue inset. (B) The distance from the center of each
lymph node volume unit (green) to the closest artery (red) and vein (blue) was
calculated in three dimensions. .……………………………………………………………..82
Figure 3.3: Histograms derived from the Visible Human Male illustrating the spatial
proximity of nodal tissue in relation to the closest edge of the nearest artery or vein for
the: a) femoral artery and vein; b) femoral artery and vein and great saphenous vein; c)
femoral artery and vein and superficial circumflex iliac vein and d) femoral artery and
vein, great saphenous vein and superficial circumflex iliac vein. ..................................87
Figure 3.4: (A). A frontal view of a three-dimensional surface rendering for a typical
patient with the left femoral artery and vein and inguinal lymph nodes delineated and (B)
the corresponding scatter plot distribution of the lymph node volume elements in relation
to the point collapse surface of the femoral artery and vein. (C) A frontal view with the
left femoral artery and vein and superficial circumflex iliac vein delineated and (D) the
xii
corresponding scatter plot distribution of the lymph node volume elements in relation to
the point collapse surface of these vascular segments. Note the antero-radial
distribution of the lymph node volume elements in relationship to the femoral vessels
and the effect of the inclusion of the superficial circumflex iliac vein on the distribution.
.......................................................................................................................................92
Figure 3.5: Inguinal nodal clinical target volume at twelve cranio-caudal levels in the
groin developed using our population-based model of lymph node topography. The
vascular expansions were trimmed to exclude bone, muscle, and skin. Legend: Femoral
arteries – red; Femoral vein – navy blue; Superficial circumflex iliac vein – light blue;
Sartorius muscle – brown; Adductor longus muscle – khaki and lymph nodes – green.
..............................................................................................................………………...94
Figure 4.1: Axial MR images for: (A) the common iliac vessels and common iliac lymph
node (red arrow); (C) the external iliac vessels and external iliac lymph node (white
arrow) and (E) the internal iliac vessels and an internal iliac lymph node (blue arrow)
and the corresponding scatter plot distributions of the lymph node volume elements in
relation to the surface of these vascular segments. Note: the red dot (0,0) denotes the
point defining the vessel surface. ...........................................................………………115
Figure 5.1: (Left) T2-w Fast Spin Echo MR image of the pelvis at the level of the
acetabulum with ( ) denoting a left lateral superficial inguinal lymph node. (Right) DWI
MR image at the level of the acetabulum with ( ) denoting a left lateral superficial
inguinal lymph node. ...………………………………………………………………………120
xiii
List of Abbreviations
AP Anterior-Posterior
CT Computed Tomography
CTV Clinical Target Volume
DICOM Digital Imaging and Communications in Medicine
ePLND extended pelvic lymphadenectomy
FOV Field of view
GTV Gross Target Volume
GU Genitourinary
GYN Gynecologic
IM Internal Margin
IMRT Intensity modulated radiotherapy
nCTV nodal Clinical Target Volume
nodal CTV nodal Clinical TargetVolume
nPTV nodal Planning Target Volume
MR Magnetic Resonance
MRI Magnetic Resonance Imaging
PA Posterior-Anterior
PET Positron Emission Tomography
POP Parallel Opposed Pair
PTV Planning Target Volume
RTOG Radiation Therapy Oncology Group
S3 Third Sacral Vertebra
SAD Short-axis diameter
xiv
SM Set-up Margin
T Tesla
T2-w T2-weighted
232Th Thorium
US Ultrasound
USPIO Ultra-small Particles of Iron Oxide
VHM Visible Human Male
1
Chapter 1. General Background and Literature Review
Science is the belief in the ignorance of experts.
- Richard Feynman
1.1 Lymphatic Anatomy and Function
The lymphatic system functions within the immunologic system, playing a key role in
host defense, in addition to the absorption and transportation of nutrients from the
gastrointestinal system and the regulation of interstitial fluid volume. The lymphatic
network arises embryologically from the developing blood vessels and functions in
parallel with the cardiovascular system1. The arterial system acts as a conduit to
distribute oxygen and other nutrients throughout the body to support tissue growth,
development and maintenance. The venous system returns the waste-laden blood to
the kidneys, liver and lungs where the products of metabolism are processed and/or
eliminated. The fluid that remains within the interstitial space enters the lymphatic
system and passes through a network of lymphatic vessels and nodes to ultimately
empty into the venous system before reaching the heart.
The lymphatic system architecture is similar to the venous system, with a fine capillary
network permeating most tissues and transitioning to successively larger lymphatic
vessels1. Lymphatic vessels possess three layered walls comprised of intima, media
and adventitia, as do the veins. Unlike the veins however, the lymphatic vessels tend to
2
be of much smaller diameter and are difficult to appreciate without the use of
specialized imaging techniques2, 3.
Lymph nodes of varying sizes are distributed along the routes of lymphatic return. They
are glandular structures, which vary markedly in both size and shape and are comprised
of a peripheral cortex and a medullary core (Figure 1.1). The parenchyma is surrounded
by a collagenous fiber capsule immediately beneath which is the subcapsular sinus.
Trabeculae extend from the subcapsular sinus to the interior and a coarse network of
reticular fibers that create an interdigitating sinus system. As with the lymphatic
vessels, standard clinical radiographic imaging techniques lack sufficient resolution to
detect normal, non-diseased lymph nodes and are unable to discern their inner
structure.
Communicating with the intercellular space, in which interstitial fluid resides, the
meshed capillary networks afford entry into the lymphatic system2, 4. Lymph arises from
interstitial fluid and enters into the blind-ended sacs at the terminus of a given capillary
network. Once in the lymphatic network, the lymphatic fluid progresses through
lymphatic collectors to reach the lymphatic vessels. The initial lymphatic capillaries,
collectors, vessels and nodes that communicate and direct lymph within a particular
anatomic region together constitute a lymphatic drainage basin. Adjacent lymphatic
basins are often interconnected in complex ways.
Fluid enters the lymphatic network through the route that presents the least amount of
resistance. Under normal physiologic conditions, the fluid within a territory flows to the
3
lymphatics in closest proximity. The system however is complex and lymphatic flow in
one region often influences flow in adjacent regions (Figures 1.2, 1.3 and 1.4)5, 6. For
example, efferent lymphatic vessels, which act as conduits for lymph exiting a lymph
node within a given lymphatic basin, may function as afferent vessels to a node within a
different lymphatic basin These interconnections produce a structural physiologic
reserve that is important during times of increased interstitial fluid load.
Lymph nodes are active in the immunologic filtration of lymph and the generation and
storage of lymphocytes. During embryogenesis, the volume of nodal tissue involved in
the lymphatic drainage for a given region is formed. With growth and development, this
volume may be divided into several small volume lymph nodes or a small number of
larger volume lymph nodes all of which occupy the fatty tissue around the blood
vessels7. As individuals age, the medullary cortex within a lymph node may involute
resulting in senile atrophy. While the nodal capsule remains, the node itself may
become difficult to detect clinically5. When an immune response is initiated, these nodes
may return to function with their peers in a lymphatic basin. Irrespective of the presence
or degree of atrophy, lymph will still continue to pass through all of the lymph nodes
within a given lymphatic basin.
The histologic architecture of the lymph node interior facilitates its ability to filter lymph
fluid. In a given lymphatic territory, lymph will enter into the node through an afferent
vessel and empty into the subcapsular sinus. Once in the lymphatic interior, the lymph
travels through only a part of the nodal interior with the trabeculae, cortical and
medullary sinuses effecting lymph flow direction (Figures 1.1 and 1.4)6. Metastases
4
entering the subcapsular sinus through an afferent vessel will not be dispersed
throughout the entire marginal sinus but instead enter into the porous intermedullary
sinuses in proximity to their point of entry into the lymph node. Nodal filtration during the
period following metastatic seeding is not markedly impaired as only one region is
initially affected. With on-going growth and proliferation, cancer cells may extend
throughout the nodal interior and restrict fluid entry by destroying or effacing the lymph
sinuses.
5
Figure 1.1: Diagrammatic representation of a lymph node in cross-section showing the
entry and passage of lymph, its blood supply and the distribution of macrophages within
its interior.
6
Figure 1.2: Representation of lymph (blue and yellow) draining from a lymph territory
(blue) into a first echelon lymph node shown in cross-section.
7
Figure 1.3: Schematic depiction of the interconnections between lymphatic capillaries,
collectors, vessels and nodes for adjacent lymphatic territories. (Modified from 5, 6)
8
Figure 1.4: Schematic of the routes of lymph drainage from the lymphatic territories and
the proportion of the territory specific lymph in the lymph nodes downstream. (Modified
from 6).
9
Lymph entering peripherally into the lymphatic capillaries will eventually reach a central
location of venous return through either the thoracic duct, which empties into the left
brachiocephalic vein, or the right lymphatic duct, which empties into the right subclavian
vein. Additional lymphaticovenous or lymphovenous anastomoses have been
described8-18 but are infrequent and tend not to function under normal physiologic
conditions. The purpose, frequency and distribution of these anastomoses remains
unclear.
The form of the lymphatic system is such that anatomic and/or physiologic disturbances
may transpire without markedly compromising its function. Were an obstruction to occur
distally, the lymph within the intercellular space would continue to drain to that site
which presented the least amount of resistance. In the region of the obstruction, the
fluid instead of entering into the obstructed capillary network adjacent to it travels in the
direction of lowest pressure and enters into the lymphatic system at some distance from
its original location. Similarly, obstruction of lymphatic collectors and afferent and
efferent lymphatic vessels tend to alter the route and passage of lymph to what would
typically be secondary or tertiary routes of drainage under normal conditions.
Intralymphatic growth of tumor metastases will compromise the sinus system within the
node and retard lymph entry into it. With an increase in intranodal pressure, lymph will
be diverted elsewhere within the system to be filtered by other nodes. An increase in
pressure within the lymph vasculature may open previously dormant lymphaticovenous
anastomoses. All of these responses are adaptive and facilitate continued functioning of
the lymphatic system in spite of local or regional insults or stressors4.
10
An appreciation of the form and function of the lymphatic system in its normal and
aberrant states is instructive in the pathologic and radiographic assessment of lymph
node metastases. Knowledge of the lymphatic drainage of a given organ or region and
an understanding of lymphatic physiology affords an insight as to the probable sites of
initial nodal spread from cancers arising from within it. Therefore, cancer staging and
therapy is predicated upon a sound study and understanding of the lymphatic system.
Despite the importance of the lymphatic system in maintaining normal physiologic
function and its role in host defense, research into the lymphatic system lags behind its
circulatory counterpart. The single most comprehensive study of the lymphatic system,
Vasorum lymphaticorum corporis humani historia et ichnographia, was undertaken by
Paolo Mascagni19. This exhaustive cadaveric study used cannulation and mercury
injection to identify the lymphatic vessels (Figures 1.5 and 1.6). Despite the passage of
223 years since its publication, there exists no modern non-invasive or invasive imaging
equivalent. While further studies such as those of Rouviere, have helped to describe
and refine the patterns and routes of lymphatic transport through the parietal and
visceral lymph nodes, Mascagni is credited with identifying and naming approximately
50% of the known lymphatic vessels7.
11
Figure 1.5: (Left) Engraved front plate of Vasorum lymphaticorum corporis humani
historia et ichnographia by Mascagni, and (Right) apparatus used to infuse mercury into
the cadaveric specimens and depictions of the efferent and afferent lymphatics with
associated lymph nodes.
12
Figure 1.6: Representative engraved plates from Vasorum lymphaticorum corporis
humani historia et ichnographia by Mascagni depicting (Left) the inguinal, iliac and
aorto-caval lymphatic vessels and nodes within the pelvis and retroperitoneum and
(Right) the left inguinal region showing the exquisite detail inherent in this work.
13
While no comparable study has been done in the modern era, the National Institute of
Health has recorded post-mortem anatomy using digitized photomacrographs of male
and female cadavers subjected to serial axial macrotomal sectioning20. The Visible
Human Male data set that resulted from this process is comprised of axial images with
pixel dimensions of 0.14 mm x 0.14 mm and 1 mm axial slice spacing, and the Female
data set with pixels measuring 0.33 mm x 0.33 mm and 0.33 mm axial slice spacing.
The inherent natural contrast between adjacent anatomic structures facilitates both
manual and automatic segmentation21. Delineation of the structures of interest and post-
processing can provide 3-dimensional rendering of the lymph nodes, blood vessels,
organs and clinical volumes of interest (Figure 1.7). While these are high fidelity virtual
representations of lymph nodes, organs and other anatomic structures of interest, the
lymphatic system vasculature is too fine and lacks sufficient inherent contrast to be
readily discernable even at this resolution.
In contemporary clinical practice, there are no methods for the in vivo detection of the
lymphatics (lymphatic capillaries through to lymph nodes and efferent lymphatic
vessels) of the inguinal and pelvic region in their entirety. This has profound implications
for the investigation and management of individuals with disorders and diseases
involving the lymphatic system. In the absence of a comprehensive image based means
for patient specific assessment, incorporation of information that can be obtained from
nuclear medicine and diagnostic radiology studies with knowledge of lymphatic structure
and function is the norm.
14
Figure 1.7: (a) High resolution pelvic axial section from the Visible Human Anatomic
Series illustrating (b) small lymph nodes (arrows) close to the external iliac vessels. (c)
Three-dimensional computer rendering derived from this dataset showing the
relationship between 177 pelvic and para-aortic lymph nodes (green) and the adjacent
vessels (red and blue).
15
1.2 Pelvic Lymphatic Network
Pelvic tumors are among the most common malignancies in women and men and are
associated with a significant burden of suffering worldwide22-24. Many pelvic tumors
including prostate, urinary bladder, uterine, cervical, vulva and anal canal cancers
frequently spread via the lymphatic channels to pelvic and inguinal lymph nodes. Lymph
node involvement at diagnosis generally is associated with a high risk of metastatic
disease at other sites outside of the pelvis and lower overall survival rates following
treatment. In patients undergoing surgical treatment, pelvic lymph node dissection is
frequently performed at the same time as removal of the primary tumor. Lymph node
dissection may be therapeutic as well as providing valuable information about prognosis
and the need for additional treatment like chemotherapy25-48. In patients receiving
radical or adjuvant radiotherapy, pelvic and inguinal lymph nodes are often included in
the clinical target volume (CTV) with the aim of reducing recurrence and improving
survival. The success of radiotherapy in this context is dependent on a detailed
understanding of pelvic and inguinal lymph node anatomy and patterns of cancer
spread.
The pelvic lymphatics are divisible into two primary networks of lymph nodes and
connecting lymph vessels: parietal and visceral25, 26, 28-30, 49-51. The parietal network of
lymphatics receives lymph from the walls of the pelvis that drain the perineum and
muscles of the pelvic girdle. The visceral network drains the pelvic organs via a series
of vessels and nodes that are situated in close relation to the pelvic arteries and veins.
The visceral lymphatics can be divided into major groups based on their anatomic
16
relation to the pelvic vasculature: common iliac, internal iliac, external iliac, superficial
inguinal and deep inguinal (Figures 1.8, 1.9, 1.10 and 1.11). The following subsections
describe in more detail the lymphatic architecture of the regions of relevance to this
body of work.
1.2.1 Common Iliac Lymphatics
The common iliac lymph nodes (4 to 7 nodes in most people) are in close proximity to
the common iliac vessels and comprise three distinct groups: the medial, intermediate
and lateral common iliac lymph node chains49, 51. The medial chain occupies the region
on the inner aspect of the common iliac artery and forms a triangular arrangement with
the corresponding nodes of the opposite side. The intermediate chain (3 to 4 nodes) is
located on the posterior aspect of the common iliac artery and vein7. The lateral chain (2
nodes) is positioned along the lateral aspect of the common iliac artery and the medial
border of the psoas muscle4, 7, 52. The lateral and intermediate common iliac chains
receive drainage from the lateral and intermediate external iliac lymph node chains and
empty into the lateral aortic lymph node chains52. The medial common iliac lymph node
chain may receive lymphatic drainage directly from the bladder neck and cervix uteri7.
17
Figure 1.8: Frontal projection of the distal lumbar spine and bony pelvis depicting the
major vascular segments of interest.
18
Figure 1.9: Frontal oblique projection of the distal lumbar spine and bony pelvis
depicting the major vascular segments of interest.
19
1.2.2 Internal Iliac Lymphatics
The internal iliac nodes are associated with the internal iliac vessels and their
branches51. They are situated anterior to the sacro-iliac joint descending to the level of
the greater sciatic foramen. The most superior node, the superior gluteal, is located in
proximity to the origin of the superior gluteal artery4, 7, 53. The inferior most node of this
chain is situated between the umbilical and obturator arteries4, 7, 53. Intermediate nodes
may be located along the uterine, internal pudendal, inferior gluteal and middle rectal
arteries4, 7, 53. The internal iliac lymph nodes drain the pelvic viscera including the
posterior aspect of the prostate, inferior and lateral aspects of the bladder, seminal
vesicles, middle and inferior aspects of the vagina and the body of the uterus4, 7, 53.
Lymphatic drainage from the perineum, inferior prostate and lower vagina travels to the
internal iliac chain54. The efferent vessels from the internal iliac nodes ascend superiorly
through the hypogastric lamina and pass below the common iliac vein to the
intermediate group of common iliac nodes.
1.2.3 External Iliac Lymphatics
The external iliac nodes are continuous with the external iliac vessels forming three
separate chains: lateral, intermediate and medial4, 7, 51. The lateral external iliac chain is
situated within the cleft formed by the medial border of the psoas muscle and the lateral
aspect of the external iliac artery. The inferior most node of this chain (lateral lacunar
node) is situated deep to the inguinal ligament. The intermediate external iliac chain is
situated on the medial side of the external iliac artery and anterior to the external iliac
20
vein. The medial external iliac chain is situated on the medial and dorsal aspect of the
external iliac vein abutting the lateral pelvic sidewall. The external iliac lymphatics
primarily drain the lower limbs receiving lymphatic fluid from the superficial and deep
inguinal nodes4, 7, 53. Of note, the medial and intermediate chains also receive drainage
from the obturator lymphatics, which in turn receive the visceral lymphatic vessels
arising from the lateral lobes of the prostate gland, bladder fundus, cervix uteri and
upper part of the vagina53. The juxtavisceral nodes of the vagina and cervix primarily
form associations with the medial group of external iliac nodes54.
1.2.4 Superficial Inguinal Lymphatics
The superficial inguinal nodes, anterior to Scarpa’s femoral triangle, typically consist of
10 to 12 large nodes7, 49, 54. They are divided from the femoral nerve, femoral vessels
and the deep inguinal nodes by the cribiform fascia. The medial superficial inguinal
nodes receive afferent vessels from the vulva and inferior vagina54. Additional drainage
from the uterine horns, which travels through the inguinal canal to reach these nodes,
may also occur54.
1.2.5 Deep Inguinal Lymphatics
The deep inguinal lymph nodes are located medial to the femoral vein and posterior to
the fascia lata in the femoral canal49, 51. The superior most deep inguinal node may
reside in the low pelvis and has been referred to as Cloquet’s or Rosenmuller’s node7.
21
The drainage from the deep inguinal nodes is through the femoral canal to the lacunar
nodes of the external iliac chain.
1.3 Lymphatic Drainage of the Pelvic Organs
The lymphatic drainage of the prostate gland, urinary bladder, corpus uterus, cervix,
vulva and anal canal has been studied using a variety of approaches, including detailed
anatomic dissection and surgical staging of patients with cancer to define the
distribution of nodal metastases7, 49, 53-56. Typical patterns of pelvic lymphatic flow are
illustrated in Fig. 1.12. However, lymphatic drainage can be highly variable from patient
to patient, influenced by differences in the distribution of normal lymphatic vessels and
the emergence of collaterals following destruction or obstruction of the usual routes of
flow. This can occur, for example, following surgery that disrupts lymphatic vessels or
as a result of bulky cancer lymph node metastases57, 58. Nevertheless, commonalities
emerged from the anatomic and surgical studies that influenced conventional radiation
treatment planning for many years prior to the widespread implementation of highly
conformal techniques like intensity-modulated radiotherapy (IMRT).
The volumes irradiated with conventional radiotherapy field borders are typically much
greater than those defined for highly conformal treatments. While this limits the dose
that may be delivered and sparing of adjacent normal tissues, it does afford greater
assurance of target coverage. In transitioning to increasingly conformal therapies, an
22
Figure 1.10: Frontal projection of the distal lumbar spine and bony pelvis depicting the
major vascular segments of interest within the femoral triangle.
23
Figure 1.11: Diagrammatic representation of the distribution of the deep and superficial
inguinal lymph nodes within the femoral triangle.
24
understanding of lymphatic topography and routes of lymph flow is of critical
importance. Disease site-specific and patient-specific knowledge of lymphatic drainage
is becoming increasingly important to assure adequate radiotherapy targeting of critical
lymph node regions.
1.3.1 Lymphatic Drainage of the Prostate Gland
Lymphatic drainage originates within the glandular acini of the prostate, traverses the
prostatic capsule and forms lymphatic networks on the posterior and superior aspects of
the gland7, 49, 53, 55, 59-64. Lymphatic flow from the anterior lobe of the prostate occurs
primarily via the medial and intermediate external iliac chains through direct lymphatic
vessels or after passing through the prevesical nodes7, 49, 53, 55. Drainage posterior along
the course of the internal pudendal artery to the inferior gluteal nodes may also occur7,
49, 53, 55. The posterior lobe of the prostate is drained by three lymphatic trunks53. The
first arises from the base of the gland, follows the seminal vesicles, passes superior to
the terminal segment of the ureter and drains into the middle and superior nodes of the
intermediate external iliac chain. The second lymphatic pathway originates from the
posterior and inferior regions of the prostate gland and follows the prostatic artery to
empty into the internal iliac lymph nodes. The third travels posterior to the superior
gluteal, lateral sacral and promontory lymph nodes along the course of the sacro-
prostatic ligament60. The lymphatic drainage of the prostate is known to have many
anastomoses, which may result in flow through the lymphatics of the rectum, bladder,
ampulla of the ductus deferens and seminal vesicles53. Lymph from the seminal vesicles
combines with the efferent
25
Figure 1.12: Diagrammatic representation of the distribution and routes of lymphatic
drainage for the inguinal and pelvic lymph nodes. (Modified from 4).
26
lymphatic vessels of the urinary bladder and prostate to drain into the intermediate and
medial external iliac nodes as well as the internal iliac nodes.
1.3.2 Lymphatic Drainage of the Urinary Bladder
An elaborate network of lymphatic drainage is present within the mucosa and muscular
layers of the bladder. The mucosal lymphatics unite to form collecting lymph vessels
that drain initially to the prevesical space and paravesical lymph nodes before reaching
the middle and superior nodes of the medial and intermediate external iliac groups7, 53,
63, 65-67. Clearance of lymph from the anterior aspect of the bladder wall begins with
collecting lymph vessels situated in proximity to the middle third of the lateral vesicle
border. Once the lymph fluid has reached this location, it flows through vessels towards
the origin of the umbilical artery in addition to the superior vesical artery. The lymph
vessels from the posterior bladder wall unite with vessels from the lateral vesicle border
to reach the external iliac nodes. Additional lymph vessels arise from the posterior
bladder wall and course either to the posterolateral angle of the bladder and then to the
medial and intermediate external nodes or extend to the lymph vessels of the bladder
trigone. Less commonly, lymph may flow to the medial lacunar node or internal iliac
nodes. The bladder trigone predominately drains through vessels located along the
course of the uterine artery to the medial and intermediate groups of external iliac
nodes53, 68.
27
1.3.3 Lymphatic Drainage of the Corpus Uterus
Lymphatic networks exist within the mucosa of the corpus uterus in addition to the
muscularis and subserosa53, 54. Lymph vessels emerge from the lateral aspects of the
uterus to travel along the suspensory ligaments in association with the ovarian branches
of the uterine artery. Ultimately, they unite with the tubo-ovarian lymph vessels and
terminate in the lumbar nodes close to the renal and superior mesenteric arteries at the
level of the lower poles of the kidneys4, 7, 69-72. Lymphatic drainage may also occur
laterally to the external iliac lymph nodes, with associated drainage to parauterine
nodes and via lymphatic trunks in the broad ligament to the superior interiliac lymph
nodes. Drainage channels arising in proximity to the cornua, mesosalpinx and tubular
bed direct lymph to the aortic and superior gluteal nodes. Lymph vessels at the site of
insertion of the round ligament may drain to the superomedial group of superficial
inguinal nodes. Anastomoses among lymph vessels arising in the uterus may also result
in drainage into the external iliac and superior gluteal lymph nodes4, 54.
1.3.4 Lymphatic Drainage of the Cervix
A rich lymphatic network exists within the uterine cervix and the adjacent parametrial
tissues, which terminates in three principal collecting lymphatic vessels54, 63, 69, 71, 73-76.
They include: the preureteral collecting vessels found anteriorly; the retroureteral
vessels; and the uterosacral vessels posteriorly7, 54, 73. The anterior preureteral lymph
vessels course in close association to the uterine artery and travel anterior to the
ureters. They comprise the primary lymphatic chain of the cervix and terminate in the
28
superior and middle nodes of the medial and intermediate external iliac chains. Less
commonly, they may also drain into the obturator group of external iliac lymph nodes.
Traveling with the uterine vein, the retroureteral lymph vessels pass posterior to the
ureter and end in the internal iliac lymph nodal chain. Drainage to the promontory nodes
may also occur. The uterosacral collecting lymph vessels travel posterolateral to the
rectum and then superiorly to reach the lateral sacral, promontory and subaortic lymph
nodes. Uninterrupted drainage from the cervix to the aortic nodes has been observed
via vessels traveling in association with the ureter to the common iliac, subaortic and
aortic nodes at the level of the pelvic brim. The lymphatic drainage of the uterine cervix
may be direct or may be interrupted by passage through the parauterine and
paravaginal lymph nodes73. The ureterouterine node, located at the junction of the
ureter and the uterine artery, is the largest of the parauterine and paravaginal lymph
nodes. Intramural and parauterine anastomoses of the cervical lymphatics occur with
the lymphatic vessels of the corpus uterus and vagina.
1.3.5 Lymphatic Drainage of the Vulva and Anal Canal
Lymphatic drainage from the vulva typically proceeds to the medial superficial inguinal
lymphatics before reaching the deep inguinal lymph nodes and then the external iliac
lymph nodes7, 53-56. Vulvar lesions in the mid-line, in proximity to the clitoris, may drain
to either or both sides and can drain directly into the external iliac lymph nodes
effectively bypassing the inguinal ones. Similarly, posterior lesions may drain directly
into the sacral lymph nodes. Lymph from the anal canal may drain into either the
29
superficial inguinal lymphatics or through the lower rectal lymphatics and the internal
iliac and inferior mesenteric lymph nodes7, 53, 55.
1.4 Imaging of Lymphatics
The variable nature of the lymphatic drainage from the prostate gland, urinary bladder,
corpus uterus, cervix, vulva and anal canal implies the need for patient-specific
lymphatic imaging to define individualized target volumes for highly conformal radiation
treatment. Unfortunately, the fine reticular structure of the small lymph vessels and
nodes, which comprise a significant portion of the lymph vascular system, limit the
usefulness of currently available conventional imaging approaches.
Early radiographic visualization of the lymphatic system followed 35 years after
Roentgen’s discovery of x-rays. In 1930 and 1931, Funaoka and Carvalho each injected
thorium (232Th) dioxide into the subcutaneous tissues and lymph nodes to demonstrate
them radiographically77, 78. While images were obtained from this technique, the
unstable 232Th nuclide decays by alpha particle emission making the imaging agent
highly carcinogenic. It was not until 1952, when Kinmonth used a water-soluble contrast
media that was injected into cannulated lymphatic vessels to study lymphedema, that a
practicable means of imaging became available52. Injection of contrast material was
preceded by subcutaneous injection of a vital dye to opacify the small lymphatic
channels at the local site and facilitate their cannulation52. While the lymphatic vessels
could be demonstrated, the water-soluble contrast agents were limited in their ability to
visualize the lymph nodes because of rapid dilution and clearance. Malek in 1959
30
revised the technique developed by Kinmonth and employed an oil-soluble iodized
contrast material that remained stable in tissue for longer periods79. Prolonged retention
within the lymphatic system enabled oil-based media to delineate a far greater extent of
the lymphovascular system along the drainage path from the site of injection.
Cannulation of the lymphatics in the distal lower extremity allowed depiction of
lymphatic channels and nodes in the pelvis and abdomen. In addition,
lymphangiography allowed visualization of the internal structure of lymph nodes, and
the detection of small metastases that appeared as filling defects on an otherwise
uniform background of contrast uptake. Lymphangiography was a major step forward
compared to earlier imaging techniques, and was used clinically for many years to
delineate lymph nodes in patients with cancer and other ailments.
While lymphangiography had many advantages, it also had limitations that largely were
imposed by the anatomic structure and architecture of the lymphovascular system58.
Cannulation of the lymphatic vessels was invasive, the injection of the agent was
uncomfortable and needed to be done over an extended period and the distribution of
the agent in tissue was determined by patterns of lymphatic drainage from the site of
injection. Clinically relevant lymph nodes anatomically and functionally remote from the
primary drainage pathway often were not visualized. For example, the internal iliac
lymph nodes, to which central pelvic tumors frequently metastasize, were difficult to
discern following injection of lymphatic channels in the leg that drain mainly via the
inguinal and external and common iliac pathways2, 49, 52.
31
With the emergence of cross-sectional imaging, the routine use of direct
lymphangiography waned. Technologic improvements in ultrasound (US), computed
tomography (CT) and magnetic resonance imaging (MRI) supplanted the routine use of
lymphangiography in the provision of clinical care and in research. Ultrasound can
detect imaging features indicative of macrometastases in superficial lymph nodes,
including irregularities in external contour, enlargement, round shape and loss of the
central hilum80. Doppler flow can also provide useful ancillary information. Ultrasound
imaging does however lack a means of providing easily interpretable serial image data
sets. CT and MRI, while non-invasive cross sectional imaging modalities, do not have
sufficient spatial and contrast resolution to detect all of the lymphatic channels and
normal lymph nodes. Neither technique is able to identify nodal metastases less than a
few mm in size81-86. Evolving dynamic CT and MR imaging approaches may improve the
detection of small volume lymph node metastases87-89. While these modalities have
unique strengths in lymphatic imaging, they fail to capture completely the information
that what was previously available from lymphangiography.
Molecular imaging approaches provide the promise of a new paradigm. The non-
invasive measurement of patient specific tumor physiology has the potential to improve
staging and response assessment following radiotherapy and other anti-cancer
treatments and facilitate the implementation of new molecular targeted therapeutic
strategies. The use of the glucose analogue 18F-fluoro-2-deoxy-d glucose (FDG) with
Positron Emission Tomography (PET) has been shown to improve the detection of
lymph node metastases in many malignancies90-109, reflecting the higher glucose uptake
and glycolytic rates in tumors compared to most normal tissues110. A disadvantage of
32
PET is low spatial resolution that can partially be overcome by co-registration with CT
images to enhance the localization of lymph node metastases109. However, PET is
limited by an inability to consistently detect disease in sub-centimeter lymph nodes and
micrometastatic lymphatic spread111.
1.4.1 USPIO-Enhanced MR Lymphatic Imaging
A novel class of MR imaging agents that capitalizes on the properties of iron within a
magnetic field has been explored clinically112-122. Dextran-coated ultra-small,
superparamagnetic, iron oxide particles (USPIO) have been evaluated as MR lymph
node contrast agents for discriminating between malignant and benign lymph nodes.
The particles are injected intravenously and taken up by macrophages in the
reticuloendothelial system, most of which reside in lymph nodes. The iron particles
cause a local change in tissue susceptibility, which results in a signal void or dark region
on post-contrast T1-weighted and T2-weighted MR images. Uniform signal loss is
apparent in normal lymph nodes as well as in those that are enlarged due to
inflammation. However, metastatic tumor in lymph nodes appears as a persistent bright
focus on a darker background in post-contrast scans, reflecting reduced regional
macrophage infiltration and USPIO uptake in cancer (Figures 1.13 and 1.14).
The potential of USPIO as a negative contrast agent for detecting lymph node
metastases has been explored with some success. USPIO-enhanced MRI has been
shown to improve the identification of lymph node metastases in patients with pelvic
malignancies relative to standard CT and/or MR techniques. However, it has limited
33
sensitivity for the detection of metastases in lymph nodes <5 mm in size, and is unable
to identify microscopic nodal disease with the accuracy of surgical staging123. An
important advantage though, in the context of radiotherapy treatment planning, is the
capability of USPIO-enhanced MR to facilitate the identification of apparently normal
lymph nodes relative to CT or unenhanced MR117, 124. The improved identification of
these nodes could aid in the definition of lymph node target volumes for high-precision
adjuvant treatment of patients at high risk of harboring occult nodal metastases.
34
Figure 1.13: (A) diagrammatic representation of a normal lymph node in cross-section
following USPIO administration and the corresponding MR appearance of lymph node.
(B) lymph node with a moderate cancer burden and its post-contrast MR appearance.
(C) lymph node with a minimal cancer burden and its post-contrast MR appearance.
35
Figure 1.14: (a) T2-w pre-contrast axial MR image of the pelvis at the level of the
acetabulum. Inset figure depicts a normal-sized proximal inguinal lymph node with
similar signal characteristics to the surrounding tissues. (b) T2-w post-contrast MR
image showing uniform distribution of USPIO contrast agent as evidenced by the
pattern of signal loss evident in the inset image.
36
1.5 Radiotherapy of Pelvic Lymph Nodes
Radiotherapy has a proven role in the curative management of gynecological and
genitourinary malignancies125-134. It may be used as primary treatment with the intent of
eradicating gross disease or in an adjuvant fashion after primary surgery to reduce the
likelihood of recurrence in high-risk patients. Tumors of the prostate gland, urinary
bladder, corpus uterus, cervix, vulva and anal canal commonly metastasize to iliac or
inguinal lymph nodes, and irradiation of these nodes has been shown to enhance pelvic
control and/or improve patient survival39, 135-150. Historically, pelvic radiation fields that
encompassed lymph nodes were planned using general knowledge of patterns of lymph
node spread, derived from anatomic and surgical staging studies as discussed
previously. High-resolution imaging of lymph node anatomy that could be applied to
individual patients was not available, nor was the technology to allow precise radiation
treatment targeting. Therefore, treatment plans tended to be very simple with four-field,
three-field or a two-field (AP/PA POP) beam arrangements positioned using bony
anatomy. The treatment volumes generally are quite large to minimize the chance of
missing disease. However, large volumes of normal tissue were also irradiated, which
contributed to side effects and limited the dose of radiation that could be used safely151-
153.
Radiotherapy treatment planning is now largely based on CT imaging rather than planar
x-rays. CT affords an ability to obtain high contrast as well as high resolution images
with a marked improvement in the level of anatomic detail that is appreciable in
comparison to projection radiography. With this improvement in imaging, standardized
37
definitions have been developed to facilitate treatment uniformity and standardization
(Figure 1.15). The Gross Target Volume (GTV) constitutes the clinically appreciable
tumor extent with the Clinical Target Volume (CTV) being the volume that includes
microscopic spread into adjacent tissues. The CTV includes direct spread from the GTV
in addition to spread to local and regional lymph nodes (nodal Clinical Target Volume -
nCTV). A further geometric margin of expansion (Planning Target Volume - PTV) is then
applied to account for physiologic movement (Internal Margin - IM) and interfraction
differences in treatment reproducibility (Set-up Margin - SM). Derivation of a treatment
plan using these definitions will produce a 3D radiotherapy treatment volume that not
only accurately and reproducibly encompasses the key target regions but also defines
the regions that can be avoided so as to spare critical anatomical structures.
38
Figure 1.15 Depiction of radiotherapy target volumes: GTV - Gross Target Volume, CTV
- Clinical Target Volume, PTV - Planning Target Volume, IM - Internal Margin, and SM -
Set-up Margin.
39
The use of planning CT has lead to improved targeting of the primary tumor but
has not substantially influenced the delineation of nodal volumes because
conventional CT and MRI scans are imperfect in their ability to detect pelvic
lymph nodes that are not enlarged29, 154, 155. However, the majority of the pelvic
lymph nodes lie in close proximity to adjacent blood vessels. Unlike the lymph
nodes, the blood vessels are more readily discernable with either CT or MR
imaging. Therefore, several investigators have proposed using blood vessels as
surrogates for nodal position with the application of an appropriate margin of
expansion to derive nodal clinical target volumes (nCTV) for radiotherapy156-159.
These margins could either be isotropic (uniform in all radial directions around a
vessel cross-section) or anisotropic (non-uniform). The ideal nCTV margin would
include all clinically relevant nodal tissue, while excluding as much surrounding
normal tissue as possible.
Several studies have now demonstrated that standard pelvic radiotherapy fields based
on bony anatomy often fail to encompass all clinically relevant lymph nodes, despite
large treatment volumes160-163. Bonin et al. used lymphangiography to study the
adequacy of pelvic lymph node coverage, as defined by the Gynecologic Oncology
Group, in patients with cervical carcinoma161. Application of standard field borders
resulted in inadequate coverage of the lower, lateral external iliac lymph node in 45% of
patients. In a similar study, Finlay et al. delineated pelvic blood vessels as a surrogate
for the lymphatics in 43 patients with cervix cancer160. With standard pelvic fields, there
was inadequate lymphatic coverage at the level of the bifurcation of the common iliac
arteries in 79.1% of patients, inadequate lateral coverage of the external iliac arteries in
40
20.9% of patients, and inadequate anterior coverage of the external iliac arteries in
69.7% of patients160. In addition, 44.2% of patients were judged to have excessive
lateral coverage (unnecessarily wide anterior-posterior fields >2 cm lateral to the
external iliac artery), 14% had excessive anterior coverage and 9.3% had excessive
superior coverage. Similar studies using lymphangiograms and surgical clips have
corroborated these findings162, 163. Standard pelvic fields do not adequately treat
portions of the common and external iliac lymph nodes and may result in irradiation of
larger than necessary volumes of normal tissue. These studies did not directly address
the implications of these results on subsequent disease recurrence in the pelvis,
although the highly variable nature of lymph node metastatic spread in individual
patients implies that all potential sites of disease need to be encompassed within the
high dose radiation volume to achieve maximal benefit.
There have been major advances in radiation treatment delivery over the past several
years that have revolutionized the ability to deliver dose in a highly conformal manner to
specific targets, while at the same time excluding surrounding normal tissues. Figure
1.16 is an example of an intensity modulated radiotherapy (IMRT) plan for the treatment
of cancer of the anal canal and the inguinal and pelvic lymph nodes, illustrating steep
dose gradients at the edge of the high dose volume leading to improved sparing of the
sigmoid and high rectum, small bowel and bladder. It is anticipated, as the long term
results following IMRT delivery become available, that reductions in late toxicity will
become apparent because of improved normal tissue avoidance. Similarly, the potential
for dose escalation afforded by IMRT should facilitate improvements in local control and
cure. High precision treatments like this highlight the need for accurate primary and
41
nodal target volume delineation because of the increased potential to underdose tumor.
A comprehensive and reproducible pelvic and inguinal nCTV definition would allow the
potential of IMRT to be fully exploited to improve tumor control and reduce toxicity.
42
Figure 1.16 Conventional radiotherapy plan for a carcinoma of the anal canal
radiotherapy (top) and an intensity modulated plan (bottom) illustrating a steep dose
gradient at the edge of the high dose volume (green) and reduced dose to bladder (B)
and genitalia (G).
43
1.6 Hypothesis, Rationale and Objectives
Radiation therapy has entered a new era of image guidance and precision treatment
delivery, which demands accurate and reproducible primary tumor and nodal target
volume delineation for optimal results. The determination of nodal target volumes has
been particular challenging because of the anatomic and structural nature of the
lymphatics and the difficulties of imaging with conventional techniques. However,
USPIO-enhanced MR imaging now offers the potential of improved lymph node
delineation, thereby enabling more comprehensive anatomic studies of the lymphatic
system. The inherent limitations of USPIO contrast agents and existing MR technology
preclude the identification of microscopic lymph node metastases, which are important
therapeutic targets in patients undergoing radiotherapy for many pelvic malignancies.
Currently, neither this approach nor any other clinically applicable imaging technique
offers sufficient sensitivity to be used as a means of targeting only tumor-bearing lymph
nodes. Nevertheless, the capability of USPIO-enhanced MR to facilitate detection of
apparently normal lymph nodes (sub-centimeter size with uniformly low signal on post-
contrast imaging – Fig. 1.13) that may or may not contain microscopic tumor deposits
could be utilized to develop more comprehensive radiotherapy target volume definitions.
Ideally, every patient requiring irradiation of pelvic lymph nodes would first have a
USPIO-enhanced MR imaging study to define the lymph node target volume. This
currently is not practical, however, because of limited MR availability in many centers
and lack of a clinically approved USPIO contrast agent. An alternative is to use USPIO-
enhanced MR in a large cohort of representative patients to study pelvic lymph node
44
distribution in relation to more easily imaged anatomic surrogates like the tumor, pelvic
and inguinal vasculature, and derive robust three-dimensional population-based nodal
target volume definitions that could be adapted to individual patients. The application of
these anatomic models in clinical practice would ensure a high probability of accurately
targeting volumes at high risk of harboring disease, thus avoiding a geographic
treatment miss. In parallel, it would facilitate the avoidance of adjacent normal
structures and limit treatment toxicity in the context of a highly conformal radiotherapy
plan.
The fundamental hypothesis of this thesis is that USPIO-enhanced MR imaging will
facilitate the derivation of a clinically relevant, population-based pelvic and inguinal
lymph node target volume definition for radiotherapy treatment planning based on
anatomic surrogates for lymph node location. The thesis will focus on nCTV definitions
relevant to the treatment of prostate, bladder, uterine, cervical, vulvar and anal cancers.
These tumors drain mainly to the inguinal and/or iliac lymph nodes. Pelvic tumors that
have a significant component of drainage to mesenteric lymph nodes, such as rectal
cancer, will be excluded because of the greater potential mobility of these nodes and
concerns about the applicability of this methodology.
45
The specific aims of this study are to:
1. Identify iliac and inguinal lymph nodes using USPIO-enhanced MRI
2. Define the topographic relationships and spatial probability distribution functions
for the location of lymph nodes relative to the adjacent vasculature and other
anatomic surrogates.
3. Derive population-based nCTV’s for the treatment of pelvic and inguinal lymph
nodes that can be adapted to individual patients based on the location of
anatomic lymph node surrogates.
46
Chapter 2. Pelvic Lymph Node Topography for Radiotherapy
Treatment Planning from Ferumoxtran-10 Contrast-Enhanced MR
Imaging (Published as cover feature in: Int J Radiat Oncol Biol Phys.
2009 Jul 1;74(3):844-851)
2.1 Introduction
Lymph node metastases are a common site of spread in pelvic malignancies. Patients
with a pelvic primary and lymph node metastases at diagnosis have a significantly
worse prognosis as compared to those with no evidence of lymphatic involvement.
Metastases to local and distant lymph nodes play a major role in tumor staging, therapy
and prognosis. Improvements in image-guided, intensity-modulated radiotherapy
(IMRT) treatment planning and delivery may help to overcome the adverse prognostic
significance of loco-regional lymph node metastases, and contribute to improved patient
outcome.
A barrier to the widespread implementation of IMRT for pelvic malignancies is the
absence of an objective description of lymph node locations in three-dimensional space
that is easily used for radiation treatment planning. Conventional CT and MR do not
reliably detect small pelvic lymph nodes, whether normal or abnormal, and have a low
sensitivity for identifying metastases, particularly in nodes <10 mm in short axis
dimension164-167. Anatomic and embryologic studies have demonstrated the close
developmental and spatial relationships between the lymphatics and vasculature3. The
47
pelvic vessels are readily detected with contrast-enhanced CT or MR, and have been
advocated as surrogates for lymph node location during treatment planning168-170.
However, detailed information about the spatial distribution of nodal tissue in relation to
the vessels at different anatomic levels in the pelvis is lacking.
Dextran-coated ultra-small, superparamagnetic, iron oxide particles (USPIO) have been
evaluated as MR lymph node contrast agents for discriminating between malignant and
benign lymph nodes171-173. While USPIO has the potential to improve the identification
of lymph node metastases in patients with pelvic malignancies relative to standard CT
and/or MR techniques, it is unlikely that USPIO will be able to identify microscopic nodal
metastases with the accuracy of surgical staging124. USPIO has however been shown to
facilitate the identification of normal pelvic lymph nodes relative to CT or unenhanced
MR117. The improved identification of these nodes using USPIO and MR offers the
potential of defining patient-specific nodal CTV’s in a minimally invasive manner that
accounts for the risk of microscopic disease, which cannot be imaged.
The aims of this study were to: 1) construct a high quality nodal atlas describing
probability distributions for normal and diseased pelvic lymph node number, size and
position based upon a 55 patient sample, and 2) define the potential 3D nodal CTV’s to
be irradiated and a means of accurate delineation of these volumes in a three-
dimensional representation.
48
2.2 Materials and Methods
This was a single center pilot study eligible to patients with histologically confirmed
endometrial, cervix, prostate or bladder cancer with no distant metastases. To ensure
uniformity of the sample and to control for artifacts and lymphadenopathy, all subjects
were screened to exclude: men with histologically confirmed carcinoma of the prostate
and a PSA ≤4.0 or >100; evidence of distant metastasis (M1); prior radical surgery or
cryosurgery for carcinoma of the prostate; previous radiation therapy; previous
hormonal manipulation or chemotherapy; patients with biopsy-proven lymph node
involvement; women with histologically confirmed carcinoma of the endometrium or
cervix who have undergone recent pelvic surgery with the exclusion of colposcopy,
dilatation and curettage or biopsy; men or women with histologically confirmed
carcinoma of the bladder who have undergone recent pelvic surgery with the exclusion
of TURBT; previous or concurrent cancers other than superficial basal or squamous cell
skin carcinoma unless disease free for at least 5 years; or a hip prosthesis.
2.2.1 Patient characteristics
The 55 patients included in this prospective study had a biopsy proven pelvic
malignancy. The median patient age was 62 (range 48-80) years. There were 12
women and 43 men (36 prostate cancers, 9 bladder cancers, 5 endometrial cancers,
and 5 cervical cancers). None had previously undergone pelvic lymph node dissection
or received radiotherapy or chemotherapy. The study was approved by the Research
49
Ethics Board of the University Health Network and written informed consent obtained.
The results of the study were not used in therapeutic decision making for these patients.
2.2.2 MR imaging
MR was performed using a 1.5 T magnet (GE EXCITE 1.5 T, Waukesha, WI, USA) with
a four-channel pelvic phased-array coil. Patients were instructed to take nothing by
mouth four hours prior to the study. No other preparation was undertaken. Axial imaging
sequences were obtained before and 24-36 hours after contrast administration at 3 mm
intervals through the pelvis. The pulse sequences consisted of T2w FSE (TR/TE
6500/80; field of view 24-28 cm), and T2*w dual echo GRE (TR/TE1/TE2 1400/14/21
ms; field of view 26-28 cm). Pixel and voxel sizes ranged from 0.9 to 1.2 mm2 and 2.7 to
3.6 mm3 respectively. Motion artifacts from peristalsis were minimized with the
administration of hyoscine-N-butylbromide (Buscopan) 30 mg or glucagon 1 mg i.v. or
i.m. prior to each of the MR imaging sessions. MR imaging remained uniform with no
changes in equipment or technique over the duration of the study period.
2.2.3 Ferumoxtran-10
Ferumoxtran-10 was reconstituted from a lyophilized powder. A dose of 2.6 mg Fe/kg
was dissolved in 100 ml of normal saline (0.9% NaCl) and infused intravenously.
50
2.2.4 Image analysis
Analysis of the DICOM images before and after contrast infusion was performed using
3D-DOCTOR® (Able Software Corp., Lexington, MA). The lymph nodes, pelvic
vasculature, as well as musculoskeletal landmarks for the sub-aortic/presacral space
and the obturator fossa/lateral pelvic sidewall were manually delineated by a single
observer (R.D.) and reviewed by a radiologist. Lymph node size evaluation was
undertaken using the T2w FSE image series as opposed to the T2*w dual echo GRE
which is subject to USPIO induced susceptibility artifacts. To verify the identification and
anatomic accuracy of the manual operator (R.D.) defined regions of interest (vascular
segments and lymph nodes) an expert abdominal-pelvic radiologist (M.H.) reviewed 10
(18%) of the image datasets. No noted variation between the observers was apparent
with regards to either nodal identification and delineation or vessel segment
identification. Voxels defining the edge of each distinct vessel and node were identified
in the x- and y- planes for each axial slice (z-plane). The x-y-z coordinates defining the
segmented structures were then used to generate a 3D stacked image model.
2.2.5 Vascular segmentation
All image sets were reviewed with partitioning of the image data to define the vascular
and lymphatic structures of interest. Manual tracing of the vessel walls was undertaken
using continuous sampling with vertices recorded along the path length. A closed spline
curve was then generated by connection of the vertices. Object specific boundaries on
each axial slice were integrated through the series to define a three-dimensional
51
volume. All points (x, y, and z) at the boundary edge and those pixels enclosed by these
points were considered to be part of the object. Those points and pixels not part of a
target structure were considered to be background. Window width and window level
were standardized to ensure optimization of contrast and brightness.
Segmentation of the pelvic arterial and venous system was performed in a caudal
direction beginning from the abdominal aorta at the origin of the inferior mesenteric
artery. At bifurcations, the first axial section that did not contain any part of the "mother
vessel" was described as the beginning of the "daughter vessel" (Figures 2.1). The T2w
FSE, T2* GRE and T1w image series were reviewed for each patient to identify the
points of bifurcation. The paired external iliac artery and vein were delineated inferiorly
to the level where the inguinal ligament traversed the medial aspect of the artery. To
ensure consistency among patients, segmentation of these structures was continued to
the superior aspect of the femoral heads in all cases.
Vessel topology, course, segment length, and bifurcation location varies within and
between individuals. Bifurcations of the iliac vessels on the right and left sides do
demonstrate varying degrees of spatial asymmetry. Similarly, the vessel bifurcation
angle may occur through a broad range. Vessel courses, lengths, curvatures, and
branching angles will all impact upon object appreciation and segmentation accuracy.
Given that this limitation exists within current clinical radiotherapy treatment planning, its
presence within this study will not limit the clinical applicability of the results.
52
Figure 2.1 A to D: Cranial to caudal axial T2w FSE, T2* GRE and T1w images superior
(A) to inferior (C and D) the bifurcation of the abdominal aorta. (Red arrow denotes the
division of the aorta in the right and left common iliac arteries).
53
2.2.6 Lymph node nomenclature and segmentation
Following administration of ferumoxtran-10, lymph nodes were detected by a decrease
in signal on the dual gradient-echo images. Lymph nodes were classified into anatomic
groups according to the following definitions, as illustrated in Figure 2.1:
• Distal para-aortic: Adjacent to the aorta and IVC from the inferior mesenteric
artery to the bifurcation of these vessels.
• Sub-aortic/presacral: Inferior to the sacral promontory over the mid-portion of the
sacrum, medial to the sacral foramina and posterior to the peritoneum and the
posterior rectal surface.
• Common iliac: Adjacent to the common iliac vessels from the aortic bifurcation to
the branching of the internal iliac artery.
• Internal iliac: Along the medial aspect of the internal iliac vessels.
• External iliac: Adjacent to the external iliac vessels inferior to the origin of the
deep inferior epigastric vessels, extending anterior to the iliopsoas muscle and
posterior to include the obturator lymph nodes within the obturator fossa.
All visible lymph nodes in these regions were contoured regardless of size and whether
or not they were judged to contain metastatic disease.
2.2.7 Musculoskeletal boundary nomenclature and segmentation
Musculoskeletal landmarks were identified for the sub-aortic/presacral and pelvic
sidewall lymph nodes. The sub-aortic/presacral landmark was the anterior aspect of the
fifth lumbar vertebrae and sacrum from the bifurcation of the aorta and IVC superiorly to
54
the inferior aspect of S3, and laterally to the sacral foramina. The pelvic sidewall
landmark was the ilium/ischium and internal obturator muscle from the bifurcation of the
common iliac vessels superiorly to the femoral canal inferiorly.
2.2.8 Measure of Proximity of Lymph Nodes to Vascular Segments
To generate a complete three-dimensional spatial representation of the lymphatic tissue
in each nodal segment, individual lymph nodes were first divided into small subunits
measuring 0.5 (x-axis) x 0.5 (y-axis) x 3 (z-axis) mm in size using an automated
algorithm (Figure 2.2). Each subunit represented 0.75 mm3 of lymphatic tissue.
Distances in 3-dimensions from the center of each nodal subunit to the closest vascular
edge (arterial or venous) were then calculated and grouped by vascular segment. Sub-
aortic/presacral lymph nodes and iliac lymph nodes along the lateral pelvic sidewall
were also localized in relation to musculoskeletal landmarks in the same manner.
Histograms were generated for each vascular segment to illustrate the spatial
distribution of nodal tissue in relation to the vessels or bony landmarks. Summary
statistics were derived from the histograms to determine the vascular expansions
required in each segment to encompass 50%, 80%, 90%, 99% and 100% of the
lymphatic tissue.
55
Figure 2.2: A three-dimensional surface rendering of the contoured pelvic lymph nodes
identified using ferumoxtran-10 in relation to the pelvic vasculature for a typical patient.
Arteries are indicated in red, veins in blue and lymph nodes in green.
Distal para-aortic
Common iliac
Sub-aortic/presacral
Internal iliac
External iliac
56
Figure 2.3: A three-dimensional surface rendering of the contoured pelvic lymph nodes
(a) and a schematic representation (b) of the method used to determine the spatial
distribution of lymphatic tissue. The distance from the center of each lymph node
volume unit (green) to the closest artery (red) and vein (blue) was calculated in three
dimensions.
57
2.3 Results
2.3.1 Lymph node frequency, size and location
A total of 1670 lymph nodes defined by 5663 separate nodal contours were detected in
the 55 patients. The median number of lymph nodes was 30 with a range of 5 to 62
(mean 30 and standard deviation 12.8). For those lymph nodes below the chosen
radiographic size criterion for metastasis (>10 mm), the median short axis size was 4
mm, with a range between 2 and 10 mm (mean 3.7 mm and standard deviation 1.2
mm). The median total nodal volume was 6650.0 mm3 (760.6-31946.9 mm3) and mean
7532.0 mm3 (standard deviation 5569.9 mm3). Table 2.1 summarizes the frequency
distribution of lymph nodes in each of the vascular segments. Nodes were relatively
uniformly distributed in the common iliac, external iliac and internal iliac regions, but
were less frequently detected in the sub-aortic/presacral region as well as the distal
para-aortic. There were no significant differences in nodal frequency, size or volume
between the left and right sides of the pelvis or between males and females.
58
Table 2.1: Pelvic lymph node frequency and distribution
Number of nodes
(Median and range) Vascular segment
Total Left Right
Distal vena cava and
para-aortic 1 (0-16)
Common iliac 7 (0-20) 3 (0-10) 3 (0-10)
External iliac 11 (1-19) 5 (1-10) 5 (0-10)
Internal iliac 5 (0-15) 1 (0-8) 3 (0-7)
Sub-aortic/presacral 2 (0-6)
59
The main purpose of this study was to define lymph node target volumes for
radiotherapy. It is unlikely, based on previous reports, that MR image enhancement with
ferumoxtran-10 will allow lymph node metastases <3 mm in size to be detected
reliably122, 124. Therefore, radiotherapy target volumes will need to encompass all lymph
nodes and not just those with imaging-evidence of metastatic disease. In addition,
lymph nodes that were enlarged and at high risk of harboring metastatic disease by
commonly accepted CT and MR size criteria (short axis diameter >10 mm) were
excluded from the histogram analysis to avoid biasing the results. This resulted in the
removal of 7 lymph nodes from the analysis in 4 of the study patients (4 left internal
iliac, 1 right common iliac, 1 right external iliac and 1 right external iliac) and represents
less than 0.5% of the nodes identified.
2.3.2 Quantitative distance histogram analysis of lymph node volume
Lymph nodes in each patient were analyzed as a function of distance from the closest
vessel. Figure 2.3 depicts representative histograms from a typical patient for each of
the vascular regions. The small branches of the internal iliac vasculature in the pre-
sacral region were not consistently visualized on MR. Therefore, the sub-
aortic/presacral lymphatic tissue was related to the closest aspect of the anterior
sacrum rather than the nearest visible vessel to avoid biasing the results towards
unusually long distances. For the same reason, obturator and medial internal iliac
lymphatic tissue along the pelvic sidewall was related to the pelvic sidewall as
previously described (see Materials and Methods).
60
The spatial distribution of nodal tissue for the 55 eligible patients is summarized in Table
2.1. On average, 90% of the lymph nodes were detected within 8.7 mm of the nearest
vessel in the distal para-aortic region, 7.3 mm in the common iliac region, 10.1 mm in
the external iliac region and 12.1 mm in the internal iliac region. However, there was
marked variability among patients. While the distribution of the nodal tissue along the
blood vessels was noted to be anisotropic and varied along the length of each relevant
vascular segment, isotropic (uniform) margins of vascular expansion were assessed
with the intent of generating a practicable method of 3D nodal CTV delineation.
The small vessels associated with the sub-aortic/presacral and obturator lymph nodes
were not consistently visualized and readily identifiable musculoskeletal landmarks were
used instead. In the sub-aortic/presacral nodal region, the mean distance anterior to the
sacrum to encompass 90% of the nodal tissue was 8.2 mm and, in the obturator region,
the mean distance from the pelvic sidewall to encompass 90% of the nodal tissue was
7.9 mm.
61
Figure 2.4: Histograms for a typical patient illustrating the spatial distribution of nodal
tissue in relation to the closest edge of the nearest artery or vein in each anatomic nodal
region: a) distal para-aortic; b) common iliac; c) external iliac; d) internal iliac; e) pelvic
side-wall f) the sub-aortic/presacral. Note the different Y-axis scales, reflecting
differences in total lymphatic nodal volume from one region to another.
62
2.3.3 Nodal CTV for pelvic radiotherapy
The main objective of this study was to define clinically practicable rules for determining
a lymph node CTV for patients receiving pelvic radiotherapy. The spatial distributions of
lymph node tissue in relation to vascular or musculoskeletal landmarks summarized in
Tables 2.2 and 2.3 suggest that different margins of expansion around these anatomic
surrogates may be optimal in different anatomic regions. The rules for nodal CTV
delineation were derived using 3D margins of expansion sufficient to cover 90% of the
nodal tissue in 90% of patients. This definition was chosen as a compromise between
nodal coverage and excessive normal tissue irradiation. The required margins of
expansion around the distal para-aortic, common iliac, external iliac and internal iliac
vessels were 12 mm, 10 mm, 9 mm and 10 mm respectively. In addition, a 22 mm
medial expansion from the lateral pelvic sidewall, contiguous anteriorly and posteriorly
with the expansions of the external and internal iliac vessels respectively, was required
to encompass the obturator and medial internal iliac lymph nodes. A 12 mm expansion
was required anterior to the sacrum from S1-S3, contiguous laterally with the expansion
of the iliac vessels, to encompass the sub-aortic/presacral nodes. Figure 2.4 illustrates
the use of these guidelines to devise a nodal CTV for pelvic radiotherapy. The anatomic
expansions were trimmed to exclude bone, muscle and fascia, as well as bowel and
bladder, because the space occupied by any of these tissues has a very low probability
of also containing nodal tissue either at the time of imaging or at later times (data not
shown).
63
Table 2.2: Spatial distribution of lymph node tissue in relation to the closest vessel in
each vascular segment
Distance of lymphatic tissue from nearest vessel
(Mean for the entire cohort and range in individual patients, mm)
Percentile 50% 80% 90% 95% 99% 100%
Distal para-aortic 4.8
(2.4-11.5)
7.6
(3.2-22.4)
8. 7
(3.6-23. 2)
9.4
(3.7-23.9)
10.4
(4.0-24.6)
11.1
(4.0-24.8)
Common iliac 4.1
(2.1-10.5)
6.2
(2.8-18.8)
7.3
(3.1-20.9)
8.2
(3.3-21.9)
9.6
(3.5-23.1)
10.4
(3.8-23.9)
External iliac 5.2
(2.4-11.5)
8.3
(3.1-14.7)
10.1
(3.4-17.4)
11.5
(3.7-19.4)
14.0
(4.2-22.9)
15.8
(4.2-26.9)
Internal iliac 7.0
(2.8-20.3)
10.0
(3.3-27.2)
12.1
(3.4-29.8)
13.2
(3.4-30.6)
14.7
(3.4-31.8)
15.0
(3.4-32.4)
Sub-aortic/pre-
sacral*
15.5
(3.6-31.1)
19.9
(5.1-36.9)
20.5
(5.9-37.7)
20.8
(6.4-38.1)
21.2
(6.7-38.6)
21.3
(6.9-38.6)
Obturator*
Lymphatic tissue in each anatomic region was localized in relation to the closet artery or
vein.
64
Table 2.3: Spatial distribution of lymph node tissue in relation to the closest vessel in
each vascular segment and musculoskeletal land markers
Distance of lymphatic tissue from nearest vessel or musculoskeletal land
marker
(Mean for the entire cohort and range in individual patients, mm)
Percentile 50% 80% 90% 95% 99% 100%
Distal para-aortic 4.6
(2.4-11.5)
7.0
(3.2-17.7)
8.2
(3.6-19. 8)
8. 8
(3.7-20.9)
9.9
(4.0-22.4)
10.5
(4.0-23.8)
Common iliac 4.0
(2.1-10.4)
6.1
(2.8-18.8)
7.2
(3.1-20.9)
8.2
(3.2-21.9)
9.6
(3.5-23.1)
10.3
(3.8-23.9)
External iliac 3.9
(2.4-5.5)
5.5
(3.1-7.8)
6.6
(3.4-10.1)
7.6
(3.7-12.3)
9.4
(4.2-15.0)
10.9
(4.2-17.8)
Internal iliac 4.6
(2.8-8.4)
6.1
(3.3-11.0)
6.9
(3.4-13.5)
7.6
(3.4-14.5)
8.5
(3.4-15.6)
8.9
(3.4-16.6)
Sub-aortic/pre-
sacral*
6.0
(3.0-10.3)
7.8
(3.5-15.1)
8.2
(3.8-16.5)
8.5
(4.2-17.2)
8.8
(4.2-17.8)
8.9
(4.2-18.2)
Obturator* 4.6
(2.6-7.2)
7.1
(3.4-12.2)
7.9
(3.6-18.9)
9.8
(3.9-19.8)
11.8
(4.3-21.1)
13.3
(4.3-22.7)
* Lymphatic tissue in each anatomic region was localized in relation to the closet
artery or vein except in the sub-aortic/pre-sacral and obturator regions, where it was
related to musculoskeletal land markers (see text).
65
2.4 Discussion
The lymphatic system is a prominent route of spread for many malignancies. Current
imaging techniques are limited in their ability to reliably detect micro-metastases in
lymph nodes. Therefore, radiation treatment of apparently normal lymph nodes is often
advocated as a means of improving local disease control and improving survival in
patients with endometrial, cervix, bladder, prostate and gastrointestinal cancer. The
current evolution towards the use of high-precision IMRT to treat pelvic lymph nodes at
risk of harboring occult disease demands accurate knowledge of the location of these
nodes for target volume definition. This study provides a comprehensive description of
the spatial distribution of pelvic lymph nodes derived from MR imaging and the use of a
lymph node contrast agent in 55 patients. The results of this study demonstrate that: 1.
ferumoxtran-10 subjectively enhances the capability of MR to detect pelvic lymph
nodes, whether involved by cancer or not; 2. for CTV definition, appropriate margins of
expansion around major pelvic vessels or musculoskeletal nodal surrogates vary
according to anatomic regions; and 3. the margins of expansion that are required
around these anatomic surrogates to reliably encompass high-risk nodes are larger than
advocated in previous studies169. A population-based model of pelvic lymphatic
topography has been presented, along with a set of clinically-relevant rules for nodal
target volume definition.
This study, in agreement with others, demonstrated that pelvic lymph nodes are
generally situated close to vessels3. This was particularly evident in the distal para-
aortic, common iliac and external iliac regions, as illustrated in Figure 2.3 for a
66
representative patient and summarized for the entire cohort in Tables 2.2 and 2.3.
However, nodal tissue in the sub-aortic/presacral and obturator regions was found at
greater distance from the major arteries and veins. These nodes are associated with
smaller branches of the iliac vessels that are too small to be visualized reliably using
MR. Therefore, the major vessels alone do not provide sufficient surrogate information
for lymph node CTV definition, and it was necessary to incorporate bony landmarks into
our population-based model of nodal topography. The model suggests that 90% of the
nodal tissue in 90% of patients will be encompassed by symmetrical three-dimensional
expansions of the distal para-aortic (12 mm), common iliac (10 mm), external iliac (9
mm) and internal iliac (10 mm) vessels, drawn in continuity with a 12 mm expansion
anterior to the sacrum and a 22 mm expansion medial to the pelvic sidewall, as shown
in Figure 2.4. The union of the major artery and vein contours should be used to form
the nodal CTV, and the resultant volume trimmed to exclude bone, muscle, fascia,
bladder and bowel, consistent with other recommendations. While population-based
models facilitate treatment planning, it is important to carefully review all relevant
imaging for each patient and adapt the nodal CTV contours to encompass overt
lymphadenopathy or nodal outliers.
There are at least two other descriptions of pelvic lymph node topography for
radiotherapy treatment planning derived from MR imaging of ferumoxtran-10169, 170. Shih
et al. mapped the center of mass of lymph nodes metastases in 18 men with prostate
cancer to a standardized three-dimensional model using anatomic landmarks170. A 2 cm
uniform margin of expansion was found to encompass 94.5%
67
(a) (b)
(c) (d)
(e) (f)
68
Figure 2.4: Pelvic nodal CTV at six cranio-caudal levels in the pelvis developed using
our population-based model of lymph node tomography. The vascular, sacral and pelvic
sidewall expansions were trimmed to exclude bone, muscle, fascia, bladder and bowel.
(a) Distal para-aortic, (b) Bifurcation of the aorta and inferior vena cava, (c) Inferior to
the bifurcation of the common iliac vessels, (d) Mid-pelvis (e) Cranial to the acetabulum,
and (f) Mid-acetabulum.
69
of the lymph nodes thought to be at risk of harboring disease, assuming a fixed nodal
diameter of 1 cm. This margin of expansion probably is excessive based on our analysis
of the spatial distribution of lymphatic tissue in 55 patients, and would unnecessarily
limit the utility of IMRT to spare adjacent normal tissues. Taylor et al., in a series of 20
women with gynecologic tumors, found a 7 mm vessel expansion to be optimal provided
that the contours were manually modified using specified rules to include potential nodal
locations at greater distances from major arteries and veins169. Despite the apparent
difference in the proposed margins of expansion between this study and ours, the
derived nodal CTV’s are actually quite similar: The manual contour modifications that
are implicit in the Taylor et al. CTV definition, in effect, further expand their 7 mm vessel
margins so that the final volumes are relatively coincident. Our approach may be less
time consuming, and less prone to error, because less revision is required for individual
cases.
The greatest limitation of this and previous studies is the ability of MR imaging with
ferumoxtran-10 to identify small lymph nodes <3 mm in size, and microscopic lymph
node metastases124, 174. The nodes comprising the presacral and internal iliac groups
are likely underrepresented by virtue of their size. Taylor et al. reported the frequency of
lymph node contours and a summation of consecutive contours through successive
axial images is required to determine the lymph node numbers identified for each lymph
node group specified169. Within their 20 patient series, a mean of 0.4 (0-2) and 7.2 (1-
22) lymph node contours were identified for the presacral and internal iliac lymph node
groups respectively. In comparison, a median 2 (0-6) and 5 (0-15) presacral/subaortic
and internal iliac lymph nodes were identified in our 55 patient series. Recognizing that
70
the presacral lymph node group in our series included subaortic lymph nodes and that
nodes within each group may lie across more than one adjacent axial plane, there
would appear to be an equivalent or greater number of lymph nodes identified in our
series with a larger sample size. Our model for nodal CTV definition was derived from
the spatial distribution of visible lymph nodes, on the assumption that these would be
representative of all nodes at risk of harboring disease. It is possible that there are small
lymph nodes at greater distances from the major vessels that were not visualized using
this approach. However, these are likely to represent a small proportion of the total
nodal volume in each anatomic region and would not substantially alter the proposed
margins of expansion, which were defined to encompass 90% of the lymphatic tissue in
90% of patients. Ferumoxtran-10, while lowering the MR threshold for the detection of
subclinical disease, does not enhance the sensitivity enough to reliably detect
microscopic metastases174. This currently limits the utility of this approach for defining
patient-specific radiotherapy volumes that target only regions of known tumor. However,
innovative cellular MR techniques using magnetic nanoparticles have been shown in
preclinical studies to detect as few as 100 malignant cells in lymph nodes, and hold
great promise for the future175.
71
2.6 Conclusion
In conclusion, the use of MR lymphography provides an objective, three-dimensional
spatial description of relevant pelvic lymph nodes in relation to easily visualized
anatomic landmarks. Radial 3D margins of expansion around the major pelvic vessels,
with the inclusion of separate margins for the sacral and medial external and internal
iliac lymph nodes, can be used to develop a population-based nodal CTV for
radiotherapy treatment planning. The use of this model in clinical practice will assure a
high probability of encompassing most of the nodal tissue at risk of harboring
metastases in most patients, while minimizing the dose to normal tissues.
72
Chapter 3. Inguinal Lymph Node Topography for Radiotherapy
Treatment Planning from Contrast-Enhanced MR Imaging
3.1 Introduction
Primary and adjuvant inguinal radiotherapy is employed in the management of
genitourinary, gynecologic and gastrointestinal malignancies. Intensity modulated
radiotherapy may permit the delivery of an escalated dose to the lymph nodes at risk of
occult metastatic spread while potentially reducing the probability of toxicity such as
lymphedema and fibrosis of normal tissues.
Adequate delineation of a nodal clinical target volume (CTV) for inguinal lymph node
irradiation is predicated upon sound knowledge of lymphatic anatomy. Unfortunately,
the location and depth of the inguinal lymph nodes has not been well defined and
inguinal nodal radiotherapy volumes are traditionally related to anatomic surrogates
such as bony landmarks or the femoral vasculature176, 177. A potential inability to
accurately define and encompass the inguinal lymphatic CTV may result in the delivery
of an inadequate dose and a geographic miss resulting in a failure to eradicate clinically
evident or subclinical disease178. Koh et al. demonstrated the importance of adequately
defining the depth of the inguinal lymph nodes in a re-analysis of patients with inguinal
nodal recurrence following treatment for vulvar carcinoma179. The use of an absolute
depth to represent the position of the inguinal nodes without regard to an individual
patient’s weight and body habitus resulted in a failure to adequately encompass the
lymph node CTV within the high dose radiation volume and subsequent high risk of
73
inguinal node failures. From that analysis, an average patient would require a
prescription depth of 6.0 cm to adequately encompass the inguinal lymphatics. However
as demonstrated by the significant range observed (minimum mean 6.1 cm and
maximum mean 17.3 cm), this depth would need to be modified for a given individual to
respect patient specific anatomic factors. A poor understanding of inguinal lymph node
target location relative to other anatomic landmarks affected this study178, 180, 181. A
related follow-up study analyzing 31 women with cervical, vaginal or vulvar carcinomas
revealed the mean depth of the most superficial inguinal lymph node to be 2.6 cm and
the mean depth of the deepest to be 5.8 cm181. These depths were dependent on body
habitus and further support the need for a patient specific plan as opposed to an
arbitrary prescription depth.
CT based treatment planning has the ability to identify the vessels within an individual
patient avoiding the potential for over or under estimating the depth of the femoral
vessels. However, CT lacks sufficient soft tissue resolution to depict the small clinically
normal lymph nodes of interest and little is known about the precise anatomic location of
the inguinal lymph nodes relative to the femoral vessels and other adjacent anatomic
structures. While knowledge of nodal topography is still incomplete, it has been known
for some time that the spatial relationship between the lymphatics and the vasculature is
one of high anatomic reproducibility and fidelity7, 19, 55. Wang et al. plotted the
topographic distribution of clinically palpable gross inguinal lymphadenopathy on a
representative fluoroscopic simulation film to derive field boundaries to encompass the
inguinal lymphatics176. While this may be suitable for two-dimensional planning, it does
not adequately convey sufficient information to generate an acceptable patient specific
74
three-dimensional radiotherapy treatment plan. For conformal and intensity modulated
radiotherapy no validated rationale for the generation of an inguinal nodal clinical target
volume is yet available.
The objective of this study was to develop a robust and generalizable CTV definition for
inguinal lymph node irradiation based on an isotropic expansion of vascular anatomy to
encompass nodal frequency and topography using MR lymphography with ferumoxtran-
10 enhanced magnetic resonance images of the lower pelvic and inguinal region. Based
on an analysis of this data, we provide a practical working CTV definition that can be
applied to patients with gynecologic, genitourinary or gastrointestinal tumors who are
deemed to be at high risk of harboring occult inguinal lymph node metastases.
3.2 Materials and Methods
This was a single center pilot study eligible to patients with histologically confirmed
endometrial, cervix, prostate (cT1-cT3) or bladder cancer with no distant metastases.
To ensure uniformity of the sample and to control for artifacts and lymphadenopathy, all
subjects were screened to exclude: men with histologically confirmed carcinoma of the
prostate and a PSA < or = 4.0 or > 100; evidence of distant metastasis (M1); radical
surgery or prior cryosurgery for carcinoma of the prostate, previous radiation, hormonal
manipulation or chemotherapy; patients with biopsy-proven lymph node involvement;
women with histologically confirmed carcinoma of the endometrium or cervix who have
undergone recent pelvic surgery with the exclusion of colposcopy, dilatation and
curettage or biopsy; men or women with histologically confirmed carcinoma of the
75
bladder who have undergone recent pelvic surgery with the exclusion of TURBT;
previous or concurrent cancers other than superficial basal or squamous cell skin
carcinoma unless disease free for at least 5 years; or a hip prosthesis. MR imaging
remained uniform with no changes in equipment or technique over the duration of the
study period.
3.2.1 Patient characteristics
From August 2004 to June 2006, 30 patients were prospectively enrolled in this study
with biopsy proven primary carcinoma of the prostate (20), bladder (5), endometrium (3)
or cervix (2). Patients who had undergone prior chemotherapy, radiotherapy or
pelvic/inguinal lymph node dissection or hernia surgery were excluded. None of the
patients analyzed for this study had clinically or radiographically detectable inguinal
nodal metastases. The median age was 64 years (range 39-82). There were 25 men
and 5 women. All patients were imaged prior to undergoing therapy for their cancer. The
local Research Ethics Board approved the study and all of the patients who participated
provided written informed consent.
3.2.2 MR imaging
MR imaging was carried out using a 1.5 T magnet (GE EXCITE 1.5 T, Waukesha, WI,
USA) with a four-channel pelvic phased-array coil. Patients were instructed to take
nothing by mouth four hours prior to the study.
Axial imaging sequences were obtained at 3 mm intervals through the pelvis before and
24-36 hours after contrast administration. Pulse sequences used consisted of a T2w
76
FSE (TR/TE 6500/80; field of view 24-28 cm), and a T2*w dual echo GRE (TR/TE1/TE2
1400/14/21ms; field of view 26-28 cm). Motion artifacts from peristalsis were minimized
with the administration of hyoscine-N-butylbromide (Buscopan) 30 mg or glucagon 1mg
i.v. or i.m. prior to each of the MR imaging sessions.
3.2.3 Ferumoxtran-10
Ferumoxtran-10 (Combidex, AMAG Pharmaceuticals; Lexington, MA) was reconstituted
from a lyophilized powder. A dose of 2.6 mg Fe/kg was dissolved in 100 ml of normal
saline (0.9% NaCl) and infused intravenously.
3.2.4 Image analysis
Image analysis was performed on archived DICOM images using a PACS radiology
workstation (OsiriX image processing software, Version 2.7.5; OsiriX Foundation,
Geneva, Switzerland)182 and 3D modeling and image processing software (3D-
DOCTOR®: Able Software Corp., Lexington, MA). A single observer (R.D.) manually
delineated the lymph nodes, inguinal and pelvic vasculature, as well as the skin and
musculoskeletal landmarks. Lymph node size evaluation was undertaken using the T2w
FSE image series as opposed to the T2*w dual echo GRE which is subject to USPIO
induced susceptibility artifacts. To verify the identification and anatomic accuracy of the
manual operator (R.D.) defined regions of interest (vascular segments and lymph
nodes) an expert abdominal-pelvic radiologist (M.H.) reviewed 10 (33%) of the image
datasets. No noted variation between the observers was apparent with regards to either
nodal identification and delineation or vessel segment identification. Points defining the
edge of each distinct vascular and nodal element were identified in the x- and y- planes
for each axial slice (z-plane)183.
77
3.2.5 Vascular segmentation
All image sets were reviewed with partitioning of the image data to define the vascular
and lymphatic structures of interest. Manual tracing of the vessel walls was undertaken
using continuous sampling with vertices recorded along the path length. A closed spline
curve was then generated by connection of the vertices. Object specific boundaries on
each axial slice were integrated through the series to define a three-dimensional
volume. All points (x, y, and z) at the boundary edge and those pixels enclosed by these
points were considered to be part of the object. Those points and pixels not part of a
target structure were considered to be background. Window width and window level
were standardized to ensure optimization of contrast and brightness.
Segmentation of the inguinal arterial and venous system was performed in a cranio-
caudal direction beginning from the external iliac artery and vein at the level where the
inguinal ligament traversed the medial aspect of the artery. At bifurcations, the first
section that did not contain any part of the "mother vessel" was described as the
beginning of the "daughter vessel ". The T2w FSE, T2* GRE and T1w image series
were reviewed for each patient to identify the points of bifurcation. Segmentation of the
venous system was performed in an analogous manner. The vessels of interest
(femoral artery and vein, superficial circumflex iliac vein, great saphenous vein,
superficial external pudendal vein and the superficial epigastric vein) were delineated
from their origin at the point of the passage of the external iliac artery and vein under
the inguinal ligament superiorly and distally to the inferior border of Scarpa’s triangle
defined by the adductor longus and sartorius muscles on both the right and left sides7.
78
Vessel topology, course, segment length, and bifurcation location varies within and
between individuals. Bifurcations of the iliac vessels on the right and left sides do
demonstrate varying degrees of spatial asymmetry. Similarly, the vessel bifurcation
angle may occur through a broad range. Vessel courses, lengths, curvatures, and
branching angles will all impact upon object appreciation and segmentation accuracy.
Given that this limitation exists within current clinical radiotherapy treatment planning, its
presence within this study will not limit the clinical applicability of the results.
3.2.6 Lymph node nomenclature and segmentation
After administration of ferumoxtran-10, lymph nodes were identified by a decrease in
signal on the gradient-echo images, which was more marked at the longer TE.
Localization was aided by combined review with T2 images. The visualized lymph
nodes were identified and divided into right and left sides as illustrated in Figure 3.1. All
of the lymph nodes lying within anatomic volume defined by the inguinal ligament, the
adductor longus muscle and the sartorius muscle, were segmented as separate
structures.
Prior to analyzing the patient data, our approach was validated using the
photomicrographs of the adult human male (Visible Human Male – VHM) from the
Visible Human Project, National Library of Medicine20. The VHM dataset is comprised of
high resolution, 1 mm, anatomic, axial images, and allows lymph nodes as small as 1 or
2 mm in size to be identified reliably. Lymph nodes in this size range may not be seen
even with ferumoxtran-10 enhanced MR images of the study subjects, and the VHM
was therefore used to assure that the nodal distribution maps and the nodal CTV
79
definition derived from the patient MR image sets were truly representative of the nodal
anatomy.
3.2.7 Quantitative distance histogram analysis of lymph node volume
The x-y-z coordinates of the segmented structures (vascular and lymphatic) were used
to compute their spatial relationships. The boundary descriptors for the pelvic
vasculature and lymphatics were compiled in one data file for each study subject and
analyzed. Each lymph node in every image set was divided into nodal volume elements
measuring 0.5 (x-axis) x 0.5 (y-axis) x 3 (z-axis) mm in dimension using an automated
algorithm in order to generate a complete 3D spatial representation of all of the
lymphatic tissue. Paired 3D distances were determined from the centre of each nodal
volume element (representing 0.75 mm3 of lymphatic tissue) to the closest vascular
edge (arterial or venous) for each vascular segment across all axial planes (Figure 3.2).
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Figure 3.1: (A) A three-dimensional image rendering of the contoured lymph nodes,
vasculature and bony anatomy derived from the VHM with a representative image
adjacent. (B) three-dimensional rendering of pelvic and inguinal vasculature (1 inguinal
ligament, 2 sartorius muscle, 3 adductor longus muscle, 4 femoral artery and vein, 5
superficial circumflex iliac vein, 6 great saphenous vein, 7 inferior epigastric vein and
the 8 external pudendal vein) and (C) the corresponding lymph nodes.
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Anisotropic and isotropic histograms describing the distribution of distances from the
centre of each nodal volume element to the nearest vessel edge for varying bin sizes
were generated for each vascular segment to illustrate the spatial distribution of nodal
tissue in relation to the associated vessels. Scatter plot displays of the nodal volume
elements were plotted on a Cartesian coordinate system where the intersection of the
abscissa and ordinate (origin) was defined as the collapse surface of the vessels
comprising the vascular segments of interest. Summary statistics were derived from the
histograms to determine the vascular expansions required in each vascular segment to
encompass 50%, 80%, 90%, 95%, 99% and 100% of the lymphatic tissue. From this,
uniform (isotropic) vascular margins of expansion were generated. These margins were
then applied to the vessels to produce an inguinal nodal CTV delimited by the adjacent
musculature, body of the pubis and skin.
3.3 Results
On each of the 1 mm axial macrotomal sections of the Visible Human Male dataset, the
inguinofemoral region bounded by the inguinal ligament superiorly, medially, and
laterally by the adductor longus and sartorius muscles were delineated. One hundred
and sixty one sections (16.1 cm) encompassed this volume. On the right, 8 discrete
inguinal lymph nodes were identified with a range in axial size of 3 to 16 mm (mean 6)
and on the left, 17 separate lymph nodes were identified with a range in greatest axial
size of 2 to 14 mm (mean 5) (Figure 3.1). The total volume of nodal tissue on the right
was 2690.3 mm3 and 3114 mm3 on the left.
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(A)
(B)
Closest distanceof each nodal volumesegment to an arteryor vein
(A)
(B)
Closest distanceof each nodal volumesegment to an arteryor vein
Figure 3.2: (A) A three-dimensional surface rendering of the contoured left inguinal
lymph nodes with a schematic representation of the method used to determine the
spatial distribution of lymphatic tissue inset. (B) The distance from the center of each
lymph node volume unit (green) to the closest artery (red) and vein (blue) was
calculated in three dimensions.
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Following delineation of the lymph nodes in the VHM, the right and left femoral artery
and vein, superficial circumflex iliac vein, great saphenous vein, superficial external
pudendal vein and the inferior epigastric vein were identified and contoured. The
distances from each vessel to the lymph nodes in three-dimensions were analyzed
individually for a given vessel and in combination with the other vessels to determine
which vessel(s) were most closely related to the adjacent lymph nodes (Table 3.1). The
femoral artery and vein, superficial circumflex iliac vein and the great saphenous vein
had the most clinically relevant associations to the adjacent lymph nodes while the
superficial external pudendal vein and the inferior epigastric vein had the least relevant
ones (data not shown).
(A)
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Table 3.1: Vessel combinations and the spatial distribution of lymph node tissue in
relation to the closest vascular segment
Distance of lymphatic tissue in mm from nearest vessel to
encompass a given percentage of the identified lymph node (LN)
volume (Vol.)
Vessels 50% of
LN Vol.
80% of
LN Vol.
90% of
LN Vol.
95% of
LN Vol.
99% of
LN Vol.
100% of
LN Vol.
F AV 13.9 17.1 18.8 20.4 25.0 28.0
F AV
GS V
11.1 15.3 17.6 20.1 25.0 28.0
F AV
SCI V
9.5 15.1 16.9 18.2 20.0 22.8
F AV
GS V
SCI V
7.4 10.8 12.7 14.3 16.6 22.3
F AV: femoral artery and vein; GS V: Great Saphenous Vein; SCI V: Superficial
Circumflex Iliac Vein.
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The high resolution (4096 x 2048 x 24 bits) color images of the axial anatomic images of
the VHM afford an opportunity for a detailed study of lymphovascular anatomy20.
Current clinically available MR and CT imaging is not able to provide this level of
anatomic detail and resolution. To approximate what may be imaged in current clinical
care, those lymph node contours, which were 3 mm or less in diameter were censored
and the remaining lymph nodes were analyzed to yield distances of 15.2, 18.0, 19.6,
21.4, 25.2 and 27.7 mm to encompass 50%, 80%, 90%, 95%, 99% and 100% of the
inguinal lymph nodes. Censoring those lymph nodes, which were greater than 3 mm in
diameter, resulted in distances of 11.7, 14.7, 16.6, 18.3, 23.7 and 28.0 mm. The
analysis of this dataset demonstrates the feasibility of using identifiable vascular
surrogates to define nodal positions and those lymph nodes which may be at the
threshold of detection or fall below it do not appear to adversely impact upon the lymph
node to vessel distance data obtained. Those nodes that are larger than 3 mm in
diameter (detectable with the current spatial resolution of clinically available MR
scanners) are related closely enough to those less than or equal to 3 mm so that the
smaller nodes would be encompassed by the values defined using the larger nodes.
Based upon the findings from the VHM analysis, the ferumoxtran-10 data were
analyzed (Figure 3.3). For the thirty patients accrued from August 2004 to June 2006, a
mean of 8 and median of 8 inguinal lymph nodes were identified within the volume
encompassing Scarpa’s triangle on both the right and left side (Range: 3-16 right and 2-
13 left) with a mean and median depth of 26.7 and 26.4 mm (9.4-49.5) on the right and
27.7 and 27.3 mm (8.1-52.9) on the left. There was no significant difference in the
number and distribution of inguinal lymph nodes between the left and right sides. The
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mean and median short axis diameters of the lymph nodes were 5.7 and 6 mm (range
3-12). Tables 3.2 to 3.4 depict the distance of the nodal volume elements from the
closest vessel edge using both isotropic and anisotropic analyses. The effect of the
inclusion of the superficial circumflex iliac vein upon the distribution of the nodal volume
elements in relationship to the surface of the nearest vessel is illustrated in Figure 3.4.
With the inclusion of the superficial circumflex iliac vein, a reduction in the isotropic
margin of vascular expansion is achieved by better capturing the true anatomical spatial
relationships of the lymph nodes to the corresponding vessel. Using only the femoral
artery and vein, the nodal tissue is distributed in a hemispheric fashion anterior to the
vessels with 90% of this tissue lying within 27.7 mm of the vessels. The congruence in
the distribution on the scatter plot with the adjacent anatomic rendering demonstrates
the absence of visible lymph nodes posterior to the femoral vessels (Figure 3.4).
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Figure 3.3: Histograms derived from the Visible Human Male illustrating the spatial
proximity of nodal tissue in relation to the closest edge of the nearest artery or vein for:
a) the femoral artery and vein; b) femoral artery and vein and great saphenous vein; c)
femoral artery and vein and superficial circumflex iliac vein and d) femoral artery and
vein, great saphenous vein and superficial circumflex iliac vein.
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Table 3.2: Spatial distribution of lymph node tissue in relation to the closest vascular segment (Femoral Artery and Vein) derived from the 30 patient series
Distance of lymphatic tissue in mm from nearest vessel to encompass a given percentage of the identified lymph node (LN) volume (Vol. )
Percentile 50% of LN Vol.
80% of LN Vol.
90% of LN Vol.
95% of LN Vol.
99% of LN Vol.
100% of LN Vol.
10th 9.7 12.8 14.3 15.3 17.1 17.9
20th 10.2 13.8 15.2 16.1 18.2 19.3
30th 10.8 14.2 16.1 18.4 20.1 21.4
40th 12.1 15.2 18.2 21.3 23.1 24.6
50th 13.5 17.3 20.4 21.7 24.7 26.2
60th 14.2 18.8 21.1 24.2 25.8 26.9
70th 15.4 20.1 22.3 25.3 27.5 29.2
80th 16.9 22.0 23.7 29.6 31.6 34.3
90th 18.8 23.7 27.7 35.4 37.8 39.5
100th 21.4 27.1 38.5 41.3 44.0 45.3
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Table 3.3: Spatial distribution of lymph node tissue in relation to the closest vascular segment (Femoral Artery and Vein and Superficial Circumflex Iliac Vein) derived from the 30 patient series
Distance of lymphatic tissue in mm from nearest vessel to encompass a given percentage of the identified lymph node (LN) volume (Vol. )
Percentile 50% of LN Vol.
80% of LN Vol.
90% of LN Vol.
95% of LN Vol.
99% of LN Vol.
100% of LN Vol.
10th 6.4 9.4 10.9 12.2 14.1 14.8
20th 6.8 9.9 11.9 14.4 16.2 17.5
30th 7.8 11.7 13.4 15.3 17.9 19.0
40th 8.2 12.7 14.9 17.1 19.3 20.7
50th 8.7 13.7 16.0 17.6 21.1 23.3
60th 9.1 14.8 17.1 20.0 22.2 24.3
70th 10.2 15.6 18.5 20.6 23.8 26.0
80th 10.8 17.0 19.2 21.9 24.8 26.6
90th 11.7 17.8 20.5 22.5 26.9 28.7
100th 13.7 18.9 21.0 22.7 37.7 40.4
90
Table 3.4: Anisotropic analysis of the spatial distribution of lymph node tissue in relation to the closest vascular segment derived from the 30 patient series
Distance of lymphatic tissue in mm from nearest vessel to encompass a 90% of the identified lymph node (LN) volume (Vol. ) in 90% of patients
Vessels ANT MED ANT ANT LAT POST LAT POST POST MED
F AV 21.1 24.3 32.1 0 0 10.1
F AV
SCI V
15.3 20.2 19.7 7.9 8.7 13.1
F AV: femoral artery and vein; SCI V: Superficial Circumflex Iliac Vein.
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Inclusion of the superficial circumflex iliac vein, which does have lymph nodes related to
its posterior aspect, results in a significant change in the distribution of the lymph node
volume elements around the vessels’ surfaces. While the bulk of the elements are still
situated antero-laterally, points are now apparent posterior to the surface effecting a
partial shift to a more circumferential distribution closer to the vessel surface (coordinate
(0,0) on the scatter plot). The radial space around the vessel surfaces was divided into
six equal parts and analysis of the distribution of the lymph node volume elements was
undertaken to determine the anisotropic distribution of the lymph nodes. Table 3.4
depicts the margin of expansion to encompass 90% of the lymph nodes in 90% of the
patients for each sextant.
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Figure 3.4: (A). A frontal view of a three-dimensional surface rendering for a typical
patient with the left femoral artery and vein and inguinal lymph nodes delineated and (B)
the corresponding scatter plot distribution of the lymph node volume elements in relation
to the point collapse surface of the femoral artery and vein. (C) A frontal view with the
left femoral artery and vein and superficial circumflex iliac vein delineated and (D). the
corresponding scatter plot distribution of the lymph node volume elements in relation to
the point collapse surface of these vascular segments. Note the antero-radial
distribution of the lymph node volume elements in relationship to the femoral vessels
and the effect of the inclusion of the superficial circumflex iliac vein on the distribution.
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3.3.1 Nodal CTV for adjuvant inguinal radiotherapy
The main objective of this study was to define a set of practical clinical rules for
determining a lymph node CTV in relation to easily visualized anatomic surrogates of
lymph node position. The margins of expansion to define the inguinal nodal CTV were
based on coverage of 90% of the nodal tissue in 90% of patients and represent a
compromise between nodal coverage and the avoidance of excessive normal tissue
irradiation (Figure 3.5)183.
An inguinal nodal CTV may be delineated by:
1. Segmenting the femoral artery and vein within Scarpa’s triangle;
2. Segmenting the superficial circumflex iliac vein within Scarpa’s triangle if a more
refined nodal CTV with greater potential sparing of adjacent tissues is desired;
3. Segment the adjacent anatomic land markers defining Scarpa’s triangle;
4. Expand the vessel contours by 3 cm if only the femoral vessels are used or 2 cm
if both the femoral and circumflex vessels are used;
5. Revise or delimit the expanded volume to exclude the adjacent musculature and
those contours extending inferior to the region within which the sartorius muscle
crosses the adductor longus muscle; and
6. Visually inspect the volumes to ensure inclusion of any potential enlarged lymph
nodes or outliers.
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Figure 3.5: Inguinal nodal clinical target volume at twelve cranio-caudal levels in the
groin developed using our population-based model of lymph node topography. The
vascular expansions were trimmed to exclude bone, muscle, and skin. Legend:
Femoral arteries – red; Femoral vein – dark blue; Superficial circumflex iliac vein –
light blue; Sartorius muscle – brown; Adductor longus muscle – khaki and lymph
nodes – green.
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3.4 Discussion
In spite of improvements in anatomic imaging and the use of novel radiologic contrast
agents, our knowledge of lymphatic anatomy and ability to identify lymph nodes in vivo
requires further improvement to more accurately identify occult nodal metastases and
delineate patient specific patterns of loco-regional spread. The inguinal lymphatics are
sites of potential spread for some gynecologic, genitourinary and gastrointestinal
malignancies. With the use of existing anatomic and surgical data to provide parameters
for a model, imaging information can be employed to generate a three-dimensional
probability atlas for nodal location in relation to adjacent vessels and other anatomic
structures. This model provides a means of defining a class solution for radiotherapy
treatment volume delineation.
The inguinal lymph nodes are situated inferior to the inguinal ligament and are divided
into superficial and deep components. The superior superficial inguinal nodes are
distributed along the line of the inguinal ligament and lie anterior to the femoral fascia
and deep to the subcutaneous fascia while the superficial inferior nodes are situated
vertically along the proximal aspect of the great saphenous vein prior to its union with
the femoral vein at the fossa ovalis (Figure 3.1). The number of superficial nodes varies
with a range of 4 to 13 reported in the literature7. The deep inguinal lymph nodes are
situated posterior to the fascia lata within the fossa ovalis (fossa ileopectinea) and lie
medial to the femoral vein within the femoral canal. Typically 1-3 lymph nodes are
identified within this region7, 49.
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Non-lymphatic anatomic structures, which can be related to the boundaries of the
inguinal region and to the inguinal nodes, are of benefit in defining and refining a nodal
CTV. In an attempt to minimize the morbidity of inguinal lymph node dissections,
several studies have assessed the inferior, lateral, and superior extents of the deep and
superficial lymph nodes184, 185. Borgno et al. reviewed the distribution of the deep
inguinal lymphatics and noted that 33% of these nodes were situated inferior to the
junction of the saphenous vein with the femoral vein and all of the nodes fell within the
boundaries defined by the fossa ovalis185. No lymph nodes were identified inferior to the
most distal margin of the fossa ovalis. Additionally, no deep inguinal lymph nodes were
identified lateral to the femoral artery. The superficial inguinal lymphatics have been
studied in order to limit the lateral extent of the surgical field required for an inguinal
lymph node dissection186. These nodes lie within the superior aspect of Scarpa’s
triangle and are closely related to the superficial circumflex iliac vasculature. As a
consequence of this close relationship, the lymph nodes are not typically found lateral to
the medial aspect of the sartorius muscle superior to the point at which it crosses the
lateral aspect of the inguinal ligament (Figures 3. 1 and 3. 2)184, 186, 187. Similarly, the
posterior most extension of the inguinal lymph nodes have been shown not to extend
beyond the point at which the sartorius muscle crosses the adductor longus muscle in
the anatomic position (Figures 3. 1 and 3. 5).
The smallest short axis distance of the lymph nodes delineated in the VHM dataset was
2 mm. While this is smaller than the slice thickness (3 mm) used in this study and raises
concerns about identifying these lymph nodes using MRI, the close relationship of these
lymph nodes to the adjacent femoral vessels would result in them being encompassed
in a nodal CTV generated using a margin of expansion derived on the basis of larger
97
visible nodes around these vessels. The adjacent lymph nodes which are of a larger
size than the slice thickness used and are apparent on the MRI images can help to
define the outer boundaries for an anatomic envelope to encompass the inguinal nodal
basin within which the smaller (<3 mm) lymph nodes may reside.
The lymphatic system arises embryologically from the venous system with a strong
anatomic and spatial relationship between the two. The topographic distribution of the
lymph nodes is influenced by the adjacent nonvascular anatomy, which influences the
spatial distribution of the lymph nodes in relation to the adjacent vessels. While isotropic
margins of vascular expansion afford a sufficiently accurate and clinically practical
means of lymph node CTV delineation, they are an over-simplification. The anisotropic
distribution reflects the impact of the musculature, bony and integumentary anatomy on
the location of the lymph nodes and shows their close association with the vessels. A
dynamic adaptive margin of expansion to account for variations in nodal distribution
through a consecutive series of axial planes would be required to fully exploit these
anatomic relationships for clinical use. An adequate approximation for this can be
obtained with an isotropic margin of expansion limited by adjacent structures. By
delimiting the isotropic nodal expansion to avoid inclusion of the muscles surrounding
the inguinal region the superior to inferior changes can be accounted for. With the aid of
magnetic resonance lymphography to facilitate identification of the lymph nodes, the
spatial relationship to the adjacent vessels has been determined and the expansions
around the vessels necessary to encompass a specified percentage of these nodes
have been quantified (Tables 3.1 and 3.2). This knowledge may be useful in
radiographic staging and in surgical as well as radiotherapy treatment planning to
facilitate nodal localization.
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The use of conformal and intensity modulated radiotherapy enables sparing of adjacent
normal tissues. This potential can be further optimized by using the adjacent
musculature to limit an isotropic margin of expansion to produce a nodal CTV that
encompasses all of the inguinal lymph nodes while eliminating the inclusion of adjacent
normal structures within the CTV (Figure 3.5). While further studies are required to
better delineate nodal patterns of failure, the nodal CTV generated using a modified 2
cm isotropic margin of expansion around the femoral artery, vein and the superficial
circumflex vein provides a volume, which is consistent with that proposed by others
while facilitating the potential for normal tissue avoidance 176, 188.
3.5 Conclusion
In conclusion, the use of MR lymphography has facilitated the construction of an
objective, three-dimensional spatial description of relevant inguinal lymph nodes in
relation to easily visualized anatomic landmarks. Radial 3D margins of expansion
around the femoral artery, vein and the superficial circumflex iliac vein were used to
develop a population-based inguinal nodal CTV for radiotherapy treatment planning.
The use of this model in clinical practice will assure a high probability of encompassing
most of the nodal tissue at risk of harboring metastases in most patients, while
minimizing the dose to normal tissues
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Chapter 4. General Discussion
Prior to the development of lymphography, no clinically practical means of lymph node
delineation for radiotherapy treatment was available. With the entry of oil-based
lymphography into clinical practice, lymphograms were used to identify lymph nodes at
high risk of harboring metastatic disease, which advanced the treatment of
genitourinary, gynecologic, gastrointestinal and other malignancies156, 163, 189-200.
However, the sensitivity of clinical lymphography for the detection of microscopic
disease is markedly inferior to surgical dissection and histologic examination. Non-
contrast and to a significant extent contrast-enhanced x-ray radiographs of this era were
unable to fully visualize relevant soft tissue anatomy including the pelvic organs, the
vasculature as well as the lymphatics. Therefore, target localization for radiotherapy
treatment planning was incomplete and imprecise, necessitating that large volumes of
normal tissues be treated to avoid missing regions of disease. In the absence of a
clinically feasible means for accurate vascular or lymphatic identification, the wealth of
anatomic information obtained up to this point in time could not be adequately translated
to the two-dimensional treatment planning radiographs nor could it be leveraged in the
routine provision of clinical care.
The development and use of non-contrast and contrast enhanced CT and MRI and the
availability of multi-planar cross-sectional imaging has enabled three-dimensional
radiotherapy treatment planning. Multi-slice imaging with CT and MRI are increasingly
able to generate three-dimensional visualizations of greater and greater anatomic detail.
While there has been an improvement in the level of resolution that can be appreciated
100
with these modalities, they remain sub-optimal in the identification of small lymph nodes
in the pelvis and groin as well as in the detection of microscopic lymph node
metastases201, 202. Functional imaging with PET scanning has the potential to improve
the detection of clinically occult lymph node metastases to some degree but remains
insensitive to micrometastatic disease203. With further improvements in each of these
modalities, the identification of anatomical features below the current limits of resolution
will become a reality and rival the detailed anatomic information that can be obtained
through surgical dissection. Similarly, the continued development of novel
lymphotrophic contrast agents augurs a new era of lymphatic target definition. Systems
of anatomic knowledge management will become increasingly important to ensure that
the emerging information is utilized to its maximal potential for radiotherapy treatment
planning as well as other endeavors. Chapters 2 and 3 outline the most comprehensive
quantitative assessment of the pelvic and inguinal lymphatics to date. An overview of
the methods that have been used in the past to record and communicate anatomic
information provides a foundation for new approaches to translating these findings into
clinical practice.
The origins of anatomical study can be traced back to ancient Egypt. The Ebers
Papyrus of 1550 BC provides a written record of ancient Egyptian medicine that
provides a description of the heart and its vessels204, 205. This represents one of, if not
the first, recorded symbolic representations of human anatomy. Following this period,
the later works of Hippocrates (460-377 BC) and Galen (129-200/217 AD) document the
medical knowledge within ancient Greek and Roman society in both symbolic and
pictorial forms206, 207. Both Hippocrates and Galen have had a profound and lasting
101
influence on Western medical thought. Roman law, during the time of Galen, forbade
human dissection and anatomic investigations were primarily limited to the study of non-
human primates, pigs and other mammals. Galen’s works would later be supplanted by
those of Andreas Vesalius (December 31, 1514 - October 15, 1564). Vesalius, unlike
Galen, faced no similar constraints and undertook careful and methodical observational
studies of human anatomy, which resulted in the publication of De humani corporis
fabrica208. Galenic anatomy, because of the limitations during that period, contained
many inaccuracies. Through a fastidious focus on detail and iterative refinements,
Vesalius was able to refute the errors in Galenic teachings and raise the standard for
medical works with the creation of his anatomic atlas207.
This initial atlas and subsequent ones provided a means of communicating knowledge
through pictorial representations as opposed to text alone. Accurate visual depictions of
human anatomy can convey the classification of spatial objects, their structural
networks (topological boundaries), the relevant part-of networks (component parts of
objects of interest) and the spatial associations between these objects and adjacent
related or unrelated objects209. Figure 3.1C depicts a three-dimensional rendering of the
arterial and venous vasculature of interest in addition to the lymphatics and relevant
musculoskeletal anatomy that readily conveys the relationships and topographic
boundaries among these structures.. The component parts are similarly appreciable in
the vasculature and its bifurcations. Anatomy atlases can facilitate the generation of
mental constructs through a representation of reality that acts to record and convey
spatial and structural complexity 210-215 and are fundamental cornerstones of
understanding and teaching. Continued revision, the incorporation of new knowledge
102
and improvements in the manner of its representation help these atlases to be better
utilized for research and clinical care delivery.
Paper, derived from the Greek for papyrus - the medium used by the ancient Egyptians,
affords primarily a two-dimensional means to record anatomical information204, 205. Early
three-dimensional anatomic representations were undertaken as evidenced by the wax
models created under the supervision of Paolo Mascagni216-218. While of high anatomic
fidelity, they were limited as a means of conveying the individual anatomic vagaries that
are present within a population and were both labor and cost intensive. More recently,
advances in computer based medical imaging and image analysis have provided a
means of developing three-dimensional digital anatomic models. These virtual atlases
have been generated for a variety of anatomic sites based on representative individuals,
as well as for populations219-222.
Neuroanatomy and functional mapping have utilized atlases with defined coordinate
systems using data derived from a carefully studied individual221. Relating features
known for a “representative” individual to a particular subject of interest is undertaken
via application of a defined set of proportionality rules using a common reference space.
For anatomic structures that do not demonstrate significant inter-subject variability, and
for which any variability that is present can be addressed through the application of
proportionality rules, the use of one single average subject is appropriate. However, a
single average subject poorly represents anatomical structures with inherent random
variability. The USPIO studies described in this work as well as the work published by
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Taylor et al. demonstrate sufficient random variability in pelvic lymphatic and vascular
anatomy to argue against the use of a single individual or small sample169, 183.
In representing structures with a large degree of inter-subject variability, such as the
pelvic and inguinal lymph nodes, a mathematical model that describes the spatial
distributions and topographic relationships derived from a large population is more apt
to provide a representative reflection of the structure of interest and facilitate
differentiation of sub-populations222. Such atlases have been both probabilistic and
statistically based. Probabilistic atlases provide an “average” depiction of some feature
of interest in reference to a standard space or co-ordinate system for a collection of
individuals. Statistical atlases depict the variability within a population and facilitate the
evaluation of factors that account for the anatomic variability within the population and
allow the shape properties of anatomic structures and the spaces between them to be
explored223. The papers by Taylor et al., Shih et al. and Dinniwell et al. demonstrate to
varying degrees the potential utility of such an approach169, 170, 183. No one study
however fully exploits the potential of either a probabilistic or a statistical lymphatic
anatomic atlas.
From a broader perspective, there is also ongoing work to develop a standardized
representation of anatomic ontology to create the Digital Anatomist Symbolic
Knowledge Base215. This work focused on the symbolic representations and spatial
relationships of anatomic structures to produce an adaptable and integrated knowledge
base of systemic and regional anatomy that can be queried at varying levels of physical
scale215. Together a model of anatomic variability and of complex relationships
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expressed through a standardized ontology would facilitate knowledge management,
the incorporation of future data and the provision of clinical care.
At present, no such model exists, with the topographic lymph node atlas developed by
Qatarneh et al. representing the closest approximation to a whole body atlas224. Unlike
the model developed by Talairach, the model based on the Visible Human Male lacks
any means of scaling the anatomic properties or features to other individuals, which
curtails its utility225. The transition from film to digital image processing in radiology has
facilitated computer based quantitative image analysis. The introduction of the DICOM
(Digital Imaging and Communications in Medicine) standard for storing digital imaging
information allows large cross-sectional anatomic image series to be rendered and
viewed in two- or three-dimensions for volumetric analysis. Two prior studies and those
presented in Chapters 2 and 3 use the digital images from USPIO-enhanced MR
imaging to derive lymph node clinical target volumes for radiotherapy treatment
planning of pelvic malignancies169, 170, 183.
The model described by Shih et al. is comprised of 18 prostate cancer patients (10
newly diagnosed and 8 with persistent or recurrent disease)170. Within the group were
69 metastatic lymph nodes with a range of 1 to 21 (median 2 mean 3.8) identified per
patient. Fourteen of the 69 lymph nodes were situated in a para-aortic distribution, and
the remaining 55 were in the pelvis. The majority of the metastatic lymph nodes (40 of
69) were ≤ 1 cm in size. Using the 69 metastatic lymph nodes, a model was generated
with the center point of each lymph node plotted in spatial relation to the adjacent
anatomy. The center point was expanded for all nodal centers by a uniform 5 mm
105
margin to yield a sphere 1 cm in diameter, which was taken to be representative of the
node. The distance from the surface of each representative nodal sphere to the surface
the adjacent blood vessels was then assessed to determine that 94.5% of lymph nodes
were encompassed with a 2.0 cm margin of expansion around sections of the common,
external and internal iliac vessels.This study has several limitations. Only “metastatic”
lymph nodes were included in the analysis and the topologic distribution of
radiographically normal lymph nodes at risk of harboring occult micrometastatic disease
was not addressed, thereby limiting this as a useful guide for defining target volumes for
adjuvant radiotherapy. All lymph nodes were assumed to be adequately represented as
1 cm spheres, which appears to run counter to earlier works examining the short-axis
dimensions of the retroperitoneal, pelvic and inguinal lymph nodes226-229.
Grubnic et al., using a sample of 12 patients who had undergone lymphangiographic,
MRI and CT studies, determined the 95th percentile maximum sort axis dimension of
pelvic lymph nodes to be: 4 mm for the common iliac and obturator, 5 mm for the
external and internal iliac 5 mm and 6 mm for the hypogastric227. Within the
retroperitoneum, the 95th percentile maximum short axis dimension was 3 mm for the
paracaval and inter-aortocaval nodes, 4 mm for the posterior caval nodes, and 5 mm for
the low left para-aortic nodes227. Normalizing all of the nodes to 1.0 cm for all nodal
stations regardless of the underlying anatomy, could have resulted in additional
adjacent normal tissue being unnecessarily encompassed in the vascular expansion
used to form the nCTV170. These potential problems have limited the clinical acceptance
of this model.
106
A related study by Taylor et al. examined 20 patients, all of whom were newly
diagnosed with endometrial or cervical carcinomas169. None of the patients was
reported to have pathological lymph node metastases. A total of 1216 nodal contours
were generated, ranging from 30-101 (median 58) in individual patients. Of these, 627
were from the external iliac region, 303 from the obturator region, 144 from the internal
iliac region and 135 from the common iliac region (under represented as only 13/20
patients were imaged with an adequate field of view). The nodal contours had a mean
short axis of 3.6 mm with a standard deviation of 1.6 mm (1.1 to 12.1 mm). Only seven
contours (0.6%) were greater than 10 mm. Following identification and delineation of the
lymph nodes, varying margins of expansion were applied to the adjacent vasculature
and the percentage lymph node coverage was calculated. It was determined that a 7
mm margin encompassed 88% of the adjacent lymph nodes. With the addition of a 10
mm presacral margin anterior to S1/S2 and an 18 mm margin along the lateral pelvic
sidewall between the external iliac volume anteriorly and the internal iliac volume
posteriorly, all demonstrable lymph node tissue could be encompassed. In a follow-up
validation cohort using a series of 10 patients with 741 identifiable nodal contours,
application of these margins resulted in inclusion of 737 of the contours. A 17 mm
sidewall margin was found to be adequate rather than the 18 mm margin proposed
originally. Despite limitations in the field of view and the absence of data concerning
nodal frequency as opposed to contours, this study is useful for its assessment of the
distribution of lymph nodes in relation to the adjacent vessels.
The model proposed by Taylor et al. represents an improvement over the one by Shih
et al. in better reflecting the differences between the distinct vascular segments and the
107
importance of considering adjacent anatomy when delineation nodal target volumes169,
170. However, difficulties arise from the manner in which the lymph node data were
reported, and this limits comparison with the existing published literature. Qaterneh, in
an analysis of the high resolution National Institute of Health Visible Human Male data
set, identified 6 internal iliac, 14 obturator, 1 promontorial, 3 sacral and 6 common iliac
lymph nodes224. Grubnic et al. reported mean nodal numbers: of 11.9 (4-18) for the
common iliac, 10.4 (1-16) for the external iliac, 4.8 (1-9) for the obturator, 5.1 (0-13) for
the internal iliac and 1.8 (0-4) for the hypogastric227. The long-axes of a significant
number of these nodes extended across more than one axial plane. However, instead
of lymph nodes, Taylor et al. reported their data using nodal contours and nodal
frequency for each site was not specified. Similarly, the short axis dimension was not
reported in a manner consistent with the published literature230-232. By convention, there
can only be one short-axis dimension per lymph node. However, in reporting nodal
contours, a median short axis distance of 3.6 mm with a standard deviation of 1.6 and a
range of 1.1 to 12.1 was described169. Lymph nodes and nodal contours were used
interchangeably in this study, which might have introduced a left skew to the distribution
by repeatedly sampling multiple “short-axes” for a given lymph node. They cannot then
be meaningfully compared to other reported series. Unlike Shih et al., the volumes
proposed by Taylor et al. did incorporate the influence of adjacent musculoskeletal and
visceral anatomy with modifications that effect a transition from an isotropic to an
anisotropic distribution169, 170. This helps to ensure adequacy of lymph node coverage
while facilitating normal tissue avoidance.
108
Both of these studies have been incorporated into expert consensus nodal volume
definitions. A Radiation Therapy Oncology Group Genitourinary (RTOG GU) expert
panel was assembled to review material from lymphographic studies and the results of
extended surgical and sentinel lymph node dissections, in addition to the studies by
Shih and Taylor233, 234. It was concluded that the patterns of prostatic lymphatic drainage
were primarily to the internal iliac, external iliac and sacral lymphatic nodal basins. The
recommendation was for a 7 mm uniform vascular expansion with pelvic sidewall strips
of unspecified width and a 10 mm strip anterior to S1-S3 to encompass the presacral
space. The RTOG Gynecologic Oncology (GYN) expert panel reviewed source material
from anatomical and surgical atlases, surgical texts, imaging references, as well as
IMRT and patient experiences to derive their target volumes235. Similar to the GU
guidelines, they recommended a 7 mm vascular expansion with a 15 mm strip anterior
to the sacrum. A further margin to unite the external and internal iliac contours along the
lateral pelvic sidewalls was recommended but not quantified. The RTOG
Gastrointestinal (GI) group, having identified an educational need for nodal clinical
target volume delineation, formed a consensus panel and each member was asked to
delineate the clinical target volume for a representative case. Using these target
volumes, 95% confidence level consensus contours were developed236. From this
analysis, it was concluded that a 7 to 8 mm margin of expansion around iliac vessels
(≥10 mm anterolaterally) and a 10 mm strip anterior to the sacrum should be used. No
specific margin of expansion was recommended for the femoral vessels. The inguinal
lymph node target volume was to be delineated as a compartment and not based on
vascular expansion. Table 4.1 summarizes the recommendation from prior studies and
the consensus definitions.
109
Table 4.1: Comparison of recommendations for nodal CTV delineation guidelines
Shih170 Taylor*169 RTOG
GU
RTOG
GYN
RTOG
GI
Common Iliac AV 20 mm 7 mm 7 mm 7 mm
External Iliac AV 20 mm 7 mm
(18 mm
Distal
Ant/Lat)
7 mm
(10 mm
Ant)
7 mm 7-8 mm
Internal Iliac AV 20 mm 7 mm 7 mm 7 mm 7-8 mm
Obturator
(Lateral pelvic sidewall)
18 mm Not
specified
?7 mm
7 mm Not
specified
Presacral space 10 mm 10 mm 15 mm ≥10 mm
*7 mm expansion encompassed 88% of lymph node contours so modification recommended
110
In the work presented in Chapters 2 and 3, unlike Shih et al., all gross nodes were
censored and no attempt was made to generate representative nodes in their place,
since the main objective was to develop clinically relevant target volumes for adjuvant
radiotherapy183. Given the small number of such nodes, it is not likely that this had any
meaningful impact on the results. As with Taylor et al., the volumes were modified to
reflect differences in each nodal vascular segment and the adjacent anatomy169, 170. In
contradistinction to Taylor et al., who determined the isotropic vascular expansion
required to include a set amount of the adjacent lymph node tissue and then modified it
to capture the remainder, the method outlined in Chapters 2 and 3 aims to capture all
lymph nodes and then modify the volume obtained to exclude adjacent non-lymphatic
anatomy. Despite the apparent difference in the proposed margins of expansion
between Taylor’s study and ours, the derived nodal CTV’s are actually quite similar. The
manual contour modifications that are implicit in the Taylor et al. CTV definition, in
effect, further expand their 7 mm vessel margins so that the final volumes are relatively
coincident169, 170. Our approach may be less time consuming and less prone to error
because less revision is required for individual cases.
The greatest limitation of our work as well as previous studies is the inability of MR
imaging with lymphotrophic USPIO’s like ferumoxtran-10 to identify small lymph nodes
<3 mm in size, and microscopic lymph node metastases. The nodes comprising the
presacral and internal iliac groups are likely underrepresented in all of these studies by
virtue of their size. Taylor et al. reported the frequency of lymph node contours and a
summation of contours through successive axial images would be required to determine
the number of lymph nodes in each nodal group169. Within their 20 patient series, a
111
mean of 0.4 (0-2) and 7.2 (1-22) lymph node contours were identified for the presacral
and internal iliac lymph node groups respectively. In comparison, a median 2 (0-6) and
5 (0-15) presacral/subaortic and internal iliac lymph nodes were identified in our 55
patient series. Recognizing that the presacral lymph node group in our series included
subaortic lymph nodes and that nodes within each group may lie across more than one
adjacent axial plane, there would appear to be an equivalent or greater number of
lymph nodes identified in our series with a larger sample size.
Our model for nodal CTV definition was derived from the spatial distribution of visible
lymph nodes, on the assumption that these would be representative of all nodes at risk
of harboring disease. It is possible that there are small lymph nodes at greater distances
from the major vessels that were not visualized using this approach. The smallest short
axis dimension of the lymph nodes delineated in the high resolution Visible Human Male
dataset was 2 mm. While this is smaller than the slice thickness (3 mm) used in this
study and raises concerns about identifying these lymph nodes using MRI, the close
relationship of these lymph nodes to the adjacent vessels would result in them being
encompassed in a nodal CTV generated using a margin of expansion derived from
larger visible nodes. The adjacent lymph nodes that are of a larger size than the slice
thickness used, and are apparent on the MRI images, can help to define the outer
boundaries for an anatomic envelope to encompass the nodal basin within which
smaller (<3 mm) lymph nodes may reside. However, these are likely to represent a
small proportion of the total nodal volume in each anatomic region and would not
substantially alter the proposed margins of expansion, which were defined to
encompass 90% of the lymphatic tissue in 90% of patients. Ferumoxtran-10, while
112
lowering the MR threshold for the detection of subclinical disease, does not enhance the
sensitivity enough to reliably detect microscopic metastases. This currently limits the
utility of this approach for defining patient-specific radiotherapy volumes that target only
regions of known tumor.
All of the inherent limitations in our work and other previous studies of pelvic lymph
node target volume definition (adequacy of anatomic coverage, knowledge of patterns
of spread, patient specific nCTV delineation) could be mitigated in whole or in part with
the use of imaging techniques and contrast agents that are more sensitive and specific
for the detection of small lymph nodes and microscopic lymph node metastases.
Unfortunately, there is no lymph node contrast agent currently available for clinical use.
In the 55 patients analyzed to determine pelvic nodal target volumes (Chapter 2) and
the 30 patients that formed the basis for the inguinal target volume definition (Chapter
3), it has been demonstrated that a population based class solution can be derived for
isotropic margins of vascular expansion. While the model would benefit from a larger
sample size to ensure sufficient power for clinical use, it is instructive in its
demonstration of the potential of isotropic margins. These margins are capable of
encompassing the lymph nodes of interest while reducing the volume of normal tissue
included in the nCTV. A reduction in the volume of normal tissue within the high dose
nCTV may help lead to a reduction in the acute and late toxicities from pelvic
radiotherapy.
A further incremental improvement might be realized with the use of non-uniform
margins of expansion or the addition of further vascular segments upon which margins
113
of expansion may be applied. Within each vascular segment, characteristic distributions
of the lymph nodes are apparent, as illustrated in Figure 4.1. The common iliac lymph
nodes appear to be closely related to the posterior aspect of the adjacent vessels while
the external and internal iliac lymph nodes are situated more circumferentially and at
greater distances from the vessels. These differences primarily arise because of the
relationship of the vessels and nodes to the adjacent musculoskeletal anatomy,
including bone and muscle. While anisotropic expansion is a potential refinement on
isotropic margins, still further improvement could be obtained with continuously variable
patient-specific margins around vessels based on more sensitive and specific nodal
imaging techniques.
The guidelines derived for pelvic and inguinal nCTV's in Chapters 2 and 3 are based
upon the paired relationship between regional lymph nodes and adjacent vascular
segments. Within the femoral triangle, lie the femoral artery and vein in addition to the
great saphenous, superficial circumflex iliac, superficial epigastric, external pudendal as
well as the lateral and medial accessory saphenous veins (Figure 1.8 and 1.9). In
exploring the spatial distribution between the lymph nodes and vascular segments, the
impact upon the histogram plot of the addition of vascular segments to the femoral
artery and vein is readily apparent (Figure 2.3). Using the femoral artery and vein alone,
a larger margin of expansion is required to encompass the lymph nodes and the centre
of balance appears to be about 15 mm. With the addition of new vascular segments, the
balance shifts left with an associated reduction in the margin of expansion required to
encompass an equivalent volume of nodal tissue. In Figure 2.3, the skewness of the
histogram shifts left with the incorporation of the great saphenous and superficial
114
circumflex iliac veins along with a slight reduction in the range. Those lymph nodes
(lateral superficial and inferior superficial) which had been encompassed by an over
expansion of the femoral artery and vein are now paired with their corresponding artery
or vein. Were it technically possible to identify, delineate and add the remaining
vascular segments, the histogram plot would assume a normal distribution with a
balance less than 10 mm and a narrow range. In an effort to ensure an accurate and
efficient means of nCTV delineation, those vessels which are difficult to appreciate in
the absence of vascular contrast or additional imaging modalities have been omitted.
Were there a clinically practical means of defining further vascular segments such as
the branches arising from the internal iliac artery and the veins and arteries within
femoral triangle, a re-analysis of the data presented in Chapters 2 and 3 could be
undertaken to generate a population based statistical library of vascular segment
expansions that could be applied to individual named vascular arterial or venous
branches. The long intravascular circulatory time of the USPIO does enable MR
arteriograms and venograms to be obtained from which those smaller vascular
branches could be reliably segmented. This would yield an iterative refinement in the
data presented in Chapters 2 and 3 in addition to a reduction in the individual vascular
margins of expansion.
A similar effect is noted in the histogram shown in Figure 2.3. In delineating only the
proximal internal iliac vessels cephalad to their origin and extending the object
delineations along the course of the superior gluteal vessels, the remaining branches of
the internal iliacs are omitted from the analysis (Figure 1.8 and 1.9). This is also the
case for the middle sacral artery. Through the delineation of anatomic land markers for
115
Figure 4.1: Axial MR images for: (A) the common iliac vessels and common iliac lymph
node (red arrow); (C) the external iliac vessels and external iliac lymph node (white
arrow) and (E) the internal iliac vessels and an internal iliac lymph node (blue arrow)
and the corresponding scatter plot distributions of the lymph node volume elements in
relation to the surface of these vascular segments. Note: the red dot (0,0) denotes the
point defining the vessel surface.
116
the lateral pelvic sidewall and the distal lumbar and anterior sacral surface, those nodes
which would require an over expansion of the common, external and internal iliac
vessels to assure their inclusion (data not shown). With the use of the land markers,
their histogram plots remain with a narrower range and a balance more reflective of the
underlying anatomy. Were those vessels that had been omitted to be delineated and
included in the analysis, the anatomic land markers for the subaortic/sacral and lateral
pelvic sidewall would confer no additional gain and could be eliminated..
117
Chapter 5. Conclusions and Future Directions
In conclusion, isotropic margins can be derived for the pelvic and inguinal vasculature.
The application of these margins in the context of adjuvant IMRT for pelvic
malignancies should enable consistency in lymph node clinical target volume
delineation while facilitating normal tissue avoidance. Derivation of anisotropic margins
of expansion may enable additional sparing of normal tissues while providing similar
lymph node coverage to isotropic margins. Analysis of a larger sample size and
validation would help to better power this model and assess its utility for clinical use.
Studies of IMRT in the treatment of pelvic malignancies have demonstrated reduced
volumes of the small bowel, rectum, bladder and bone marrow in the high dose regions,
with a resultant decrease in acute and chronic gastrointestinal toxicities132, 157, 158, 237-240.
Application of the modified isotropic or the anisotropic margins in a prospective clinical
study with IMRT would help to determine any further reduction in toxicities related to the
reduced volume of normal tissue irradiated using these nCTV margins.
At present, the limitations to patient specific USPIO imaging include: the absence of a
clinically available agent, cost of the MRI and radiology reporting time (mean time to
report one image series by an expert observer is 80 minutes with pre- and post-contrast
imaging required), and the learning curve of the reporting radiologist174. USPIO currently
is not available in North America outside of a clinical trial. The models presented here
as well as those reported by others represent means of using a population based model
for the radiotherapy treatment planning of an individual patient169, 170, 183. These nascent
118
anatomic models parallel the efforts that have recently been undertaken in other
domains to develop an animated four-dimensional model of the human body using
morphologic, functional and molecular information within a unified representation220.
The core elements of this model include: a 3-dimensional digital atlas of the human
body, ontology-based metadata support and biomedical datasets220. This will allow
spatio-temporal modeling of complex disease processes and is sufficiently evolved to
demonstrate changes over time reflecting progression of underlying pathology. In the
context of the body of work discussed here, a depiction of the spatial topology of the
lymph node distributions within the pelvis and groin as well as the pattern of local,
regional and distant metastatic spread both within and external to the lymphatics could
be incorporated into such a model. From a lymphatic atlas, composite organ systems
could then be added and integrated into a unified and dynamic digital construct. Into this
form, medical images and spatio-temporal data could be incorporated and manipulated
in four-dimensions over a range of scales from the subcellular to the that of the entire
organism..
A standard three-dimensional anatomic form affords a framework to which additional
morphologic information can be added. For example, lymphatic vessels cannot be
adequately imaged using currently available cross-sectional imaging approaches.
However, with the use of microsurgical techniques and computed tomographic
lymphangiography, the relevant efferent and afferent lymphatic vessels can be
detected241. This new information could be incorporated into a standardized three-
dimensional anatomic model to refine currently available information about the micro-
regional distribution of lymphatic vessels and lymph nodes.
119
Image data with USPIO is limited by MRI resolution. Additional complementary pulse
sequences, such as diffusion-weighted MRI (DW-MRI), may enhance the diagnostic
accuracy. DW-MRI is a non-invasive imaging technique that examines the structure of
a biologic tissue and facilitates the assessment and measurement of tissue
microstructure (Figure 5.1). The random thermal motion of water molecules, Brownian
motion, results in tissue dependent signal attenuation. This allows tissues to be
evaluated on the basis of differences in water-proton mobility within and between
anatomic structures. DW-MRI has been investigated for a variety of pathologic
conditions such as acute stroke, multiple sclerosis, abscesses and tumors242-247. More
recently, it has been used to serially monitor solid tumor response to chemotherapy
over time and identify nodal metastases248, 249. Changes in the mobility of water
molecules in a tissue afford a non-invasive means of further characterizing a given
tissue.
120
Figure 5.1: (Left) T2-w Fast Spin Echo MR image of the pelvis at the level of the
acetabulum with ( ) denoting a left lateral superficial inguinal lymph node. (Right) DWI
MR image at the level of the acetabulum with ( ) denoting a left lateral superficial
inguinal lymph node.
121
A recent report by Thoeny et al. examined the potential of DW-MRI to facilitate reader
reporting of USPIO-enhanced MR imaging studies of normal sized lymph nodes in 20
patients with bladder or prostate cancer174. The authors evaluated the potential
diagnostic synergism between USPIO and DW-MRI using a 3-T MRI with pre- and post-
USPIO imaging series. At the completion of imaging, all patients underwent an
extended pelvic lymphadenectomy and resection of the primary tumor. The addition of
DW-MRI failed to improve the overall diagnostic accuracy for lymph node metastases.
However, DW-MRI did appear to facilitate nodal identification, as the reporting time for
the combined arm was less than that for the USPIO arm (mean of 13 minutes vs 80
minutes respectively, p<0.0001).. Eight percent (2 of 26) metastatic lymph nodes were
missed by imaging. These nodes had dimensions of 1.0 x 0.2 mm and 0.7 x 0.4 mm.
This highlights an important limitation of commonly-used MR technology and pulse
sequences in relation to the detection of micrometastatic disease.
The diagnostic accuracy of CT or MR for the detection of lymph node metastases may
be improved by integrating these anatomic imaging modalities with functional and
molecular PET or SPECT imaging. This creates the potential for synchronized
acquisition of morphologic, functional and molecular information250. While historically of
limited resolution, enhancements such as pinhole SPECT have enabled the detection of
discrete nodal metastases in sites such as the axilla251. As further improvements are
made in camera design with greater intrinsic spatial resolution, the potential for
additional discrete nodal metastasis detection will increase.
122
Additional improvement in nodal detection can be obtained by increasing the inherent
contrast between the different tissues of interest (e.g. lymph node versus adjacent non-
lymphatic anatomy) with the administration of a contrast agent. Radiographic
lymphography has recently re-emerged for axillary lymphatic mapping with the
interstitial administration of commercially-available water-soluble CT contrast media into
the breast89, 241, 252-254. Unlike the current sentinel lymph node technique, 3D-CT
lymphography provides a permanent record of the route and pattern of lymphatic
drainage from the peri-tumoral injection site within the breast through to the first, second
and third order draining lymphatics. Optical fluorophores, quantum dots, have also
been used to identify the sentinel lymph nodes in animal models. They consist of
nanometer sized (5 to 20 nm) cadmium-selenium or cadmium-tellurium semi-conductor
crystals255. Quantum dots emit energy over a narrow wavelength that is determined by
their size and shape. Therefore, using quantum dots with different characteristics allows
an assessment of routes of transport in tissue. As with 3D-CT breast lymphography, the
use of quantum dots to image lymph node drainage basins is currently limited to
interstitial peri-tumoral administration as they are not taken up by the lymphatics
following intravenous infusion255, 256. Other limitations include the potential for toxicity
from the semi-conductor core elements [261] and the production of free radicals from
the optical excitation energy leading to DNA damage and cell death257. While the unique
physical properties of quantum dots give them utility in the study of animal model
systems, the current toxicities and lack of lymphotrophism are impediments to clinical
translation.
123
Adenoviruses are known to be lymphotrophic258. This may be a consequence of their
size, negative charge or some additional biological feature. Recombinant adenoviruses
have been created to act as gene delivery vectors. Imaging reporter genes have been
successfully inserted into prostate-specific adenoviral vectors and detected following
administration of the virus in an animal model259. Unfortunately, in spite of the known
lymphotrophism, only a small fraction (0.1%) of the adenoviral dose is detectable in
lymph nodes following interstitial administration. 258. This along with the potential for
incomplete gene transfer may limit its sensitivity and subsequent clinical utility.
The overall quality of all of the different imaging modalities is improving and with this
progress, the potential for individual lymph node identification and segmentation will
become more accessible in the clinic. Using high-resolution isotropic voxels and
segmentation algorithms, one can volumetrically identify the lymph nodes of interest
within a given anatomic site. Currently available commercial software allows automated
segmentation and analysis of lymph nodes following manual identification of seed points
to guide this process260. The nodes may then by analyzed in two- and or three-
dimensional visualizations using accepted means, such as those specified by the
response evaluation criteria in solid tumors231, 261. This potential has also been explored
by others 262, 263 and can be improved upon by subdividing regions, as was presented in
Chapters 2 and 3 for inguinal and pelvic lymph nodes. Region specific delimiters for the
head and neck, axilla, thorax, abdomen, pelvis and groin would enable further
refinement of these methods.
124
In summary, this study has highlighted the importance of high resolution lymph node
imaging with an appropriate contrast agent in the current era of high precision
radiotherapy. USPIO offer several advantages for the detection of lymph nodes and
lymph node metastases that can aid in radiotherapy target volume definition.. contrast
agents are not currently available for routine clinical use. However, their demonstrated
utility will help to ensure that they (or other newly emerging nodal imaging agents) will
ventually find a place in the diagnosis and treatment of cancer, leading to greater
individualization of care. In the absence of widely available nodal contrast agents for
patient-specific treatment planning, the use of class solution nodal clinical target
volumes based on contrast-enhanced high resolution nodal imaging as presented here
will form the foundation for current patient care and the development of innovative
treatment strategies in the future.
125
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