The Alphabet Soup of Radiotherapy

GTV, CTV, PTV and all that…

In a perfect world…• It would be possible to irradiate the full

“volume to be treated” in a homogeneous way (e.g., 60 Gy) with no dose to the surrounding normal tissues.

• In this ideal situation, the prescribed dose would be 60 Gy, the recorded dose would also be 60 Gy, and the reported dose (e.g., for publication, or in multicenter studies) would be 60 Gy.

In our world…• Within the target volume, the dose may vary

between rather large limits, depending on the technical conditions.– Difference between maximum and minimum

doses in the target volume often reaches 10, 15, or even 20%.

• Some normal tissues receive doses at levels that are often similar to the prescribed dose and which sometimes approach or even exceed the tolerance limits.

The problem• A dose difference as small as 5% may lead to

real impairment or enhancement of tumor response, as well as to an alteration of the risk of morbidity.– Such a 5% uncertainty in dose can easily be

introduced by different methods for reporting.

• Inadequate reporting may lead to a false interpretation of a study and to its wrongful application.

A solution• The International Commission on Radiation

Units and Measurement (ICRU) published Report 29, Dose Specification for Reporting External Beam Therapy with Photons and Electrons in 1978.– Further interpretation of the concepts of dose

specification became necessary.– Rapidly expanding use of computers in

radiotherapy, allowing for better planning and evaluation of 3D dose distributions, brought changes in clinical practice.

References• ICRU Report 50 – Prescribing, Recording

and Reporting Photon Beam Therapy (1993)• ICRU Report 62 – Prescribing, Recording

and Reporting Photon Beam Therapy (Supplement to ICRU Report 50) (1999)

In each report, the emphasis is on reporting

ICRU recommendations for describing volumes and doses

Gross Tumor Volume (GTV)• The gross tumor volume (GTV) is the

demonstrable extent and location of the malignant growth.

• This extent can be determined by palpation or direct visualization, or indirectly through imaging techniques.

• GTV cannot be defined if the tumor has been surgically removed.– An outline of the tumor bed can be substituted by

examining preoperative and postoperative images.

Clinical Target Volume (CTV)• The GTV is generally surrounded by a region of

normal tissue, which may be invaded by subclinical microscopic extensions of the tumor.

• Additional volumes may exist with presumed subclinical spread, such as to regional lymph nodes.

• These volumes are designated clinical target volumes (CTV).

• The CTV is an anatomical concept, representing the volume of known or suspected tumor.

Internal Target Volume (ITV)• ICRU Report 62 recommends that an internal

margin (IM) be added to the CTV to compensate for internal physiological movements and variation in size, shape, and position of the CTV during therapy in relation to an internal reference point.

• The volume that includes CTV with these margins is called the internal target volume (ITV).

Set-up Margin (SM)• To account specifically for uncertainties in

patient positioning and alignment of the therapeutic beams during treatment planning and through all treatment sessions, a Set-up Margin (SM) for each beam is needed.

• Uncertainties to be compensated for may vary with different anatomical directions, and thus the size of such margins depends on the selection of beam geometries.

Sources of set-up uncertainty• Uncertainties depend on different types of

factors, such as:– variations in patient positioning,

– mechanical uncertainties of the equipment (e.g., sagging of gantry, collimators, and couch),

– dosimetric uncertainties,

– transfer set-up errors from CT and simulator to the treatment unit,

– human factors.

Planning Target Volume (PTV)• The volume that includes CTV with an IM as well as

a set-up margin (SM) for patient movement and set-up uncertainties is called the planning target volume (PTV).

• To delineate the PTV, the IM and SM are not added linearly but are combined rather subjectively.

• The margin around CTV in any direction must be large enough to compensate for internal movements as well as patient-motion and set-up uncertainties.

Treated Volume• Additional margins must be provided around

the target volume to allow for limitations of the treatment technique.

• The minimum target dose should be represented by an isodose surface which adequately covers the PTV to provide that margin.

• The volume enclosed by this isodose surface is called the treated volume.

• The treated volume is, in general, larger than the planning target volume.

Irradiated Volume• The volume of tissue receiving a

significant dose (e.g., 50% of the specified target dose) is called the irradiated volume.

• The irradiated volume is larger than the treated volume and depends on the treatment technique used.

Organs at Risk (OR)• Organs at Risk (“critical normal structures”)

are normal tissues (e.g., spinal cord) whose radiation sensitivity may significantly influence treatment planning and/or prescribed dose.

• Knowledge about the sensitivity of normal tissues is derived mainly from clinical observations.– Different approaches have been proposed for

modeling of normal tissue complication probability (NTCP).

Functional Sub Units• The FSU-concept suggests that, for the

purpose of evaluation of the volume-fractionation-response, the tissues of an Organ at Risk can be considered to be functionally organized as either “serial,” “parallel,” or “serial-parallel” structures.

• Examples of tissue organization structures in the parallel-serial model:

a) a serial string of subunits (e.g., the spinal cord)b) a parallel string of subunits (e.g., the lungs)c) a serial-parallel string of subunits (e.g., the

heart)d) a combination of parallel and serial structures

(e.g., a nephron)

Planning Organ at Risk Volume (PRV)• As with the PTV, any movements of the

Organ(s) at Risk during treatment, as well as uncertainties in the set-up during the whole treatment course, must be considered.– In particular, Internal and Set-up margins can be

identified.

• This leads, in analogy with the PTV, to the concept of a Planning Organ at Risk Volume (PRV).

• Note that a PTV and a PRV may overlap.

Scenario A• A margin is added around the GTV to take into

account potential “subclinical” invasion.– The GTV and this margin define the CTV.

• An IM is added for the variations in position and/or shape and size of the CTV.

• A SM is added to take into account all the variations/uncertainties in patient-beam positioning.

• CTV + IM + SM define the PTV on which the selection of beam size and arrangement is based.

Scenario B• The simple (linear) addition of all

factors of geometric uncertainty (case A) often leads to an excessively large PTV– Incompatible with the tolerance of the

surrounding normal tissues.

• Instead of adding linearly the IM and SM, a compromise has to be sought, and a smaller PTV has to be accepted.

Scenario C• In the majority of clinical situations, a

“global” safety margin is adapted.– In some cases, the presence of ORs dramatically

reduces the width of the acceptable safety margin.

• Since the incidence of subclinical invasion may decrease with distance from the GTV, a reduction of the margin for subclinical invasion may still be compatible with chance for cure, albeit at a lower probability rate.

Dose statistics• When recording information about a dose

distribution, or comparing competing treatment plans for the same patient, a statistical description of the dose received by various volumes can be very useful.

• The simplest such statistics would include the minimum, maximum, and average doses for each identified structure.

Dose-Volume Histogram (DVH)• Display of dose distribution in the form of

isodose curves or surfaces is useful not only because it shows regions of uniform dose, high dose, or low dose but also their anatomic extent and location.

• In 3D treatment planning, this information is essential but should be supplemented by dose-volume histograms (DVH) for the segmented structures, for example, targets, critical structures, etc..

• A DVH not only provides quantitative information with regard to how much dose is absorbed in how much volume but also summarizes the entire dose distribution into a single curve for each anatomic structure of interest.

• A DVH is a useful tool for evaluating a given plan or comparing competing plans.

V20 and such

• Rancati et al, “Factors predicting radiation pneumonitis in lung cancer patients: a retrospective study”, Radiother. Oncol. 67, 275-283 (2003)

• “… percentage of the lung receiving 20, 25, 30, 35, 40, and 45 Gy (respectively V20 V45)…”

Calculation of V20

• From the previous DVHs, the total lung volume = (27 + 6 + 4 + 1990 + 1260) = 3287 cm3

• The volumes receiving 20 Gy = (27 + 162 + 586) = 775 cm3

%6.23%1003287

77520

V

Okay, so what is V95??• Olofsen-van Acht et al, “Three-dimensional

treatment planning for postoperative radiotherapy in patients with node-positive cervical cancer. Comparison between a conventional and a conformal technique.”, Strahlenther Onkol. 175, 462-9 (1999)

• “The mean volume receiving 95% or more of the prescribed dose (V95) of the small bowel…”

And the moral of the story is…?• Unlike GTV, CTV, PTV, etc., there

does not appear to be a “standard” for the definition of Vx

• Variations in usage may be related to the treatment site.

• If in doubt about the definition of a term, ask the radiation oncologist using the term.