International Journal of Radiation Oncology � Biology � PhysicsS908

Conclusions: By decreasing the Rx isodose line, OARs and normal tissues

receive less radiation dose. While an 80% Rx line also results in normal

tissue sparing compared with 90% Rx line, greater sparing will be ach-

ieved with 60 or 70% Rx line, in which case dose heterogeneity will in-

crease, but this could potentially yield an increased TCP while at the same

time lowering NTCP. On the other hand, the gain of increased TCP and

decreased NTCP needs to be weighted over possible tumor necrosis due

the larger maximum dose within the target.

Author Disclosure: Q. Zhang: None. D. Zheng: None. Y. Lei: None. J.

Driewer: None. B. Morgan: None. M. Zhang: None. S. Li: None. S.

Zhou: None. W. Zhen: None. R. Thompson: None. A. Wahl: None. C.

Lin: None. C. Enke: None.

3771Stereotactic Ablative Radiation Therapy for GynecologicalMalignancies in the Oligometastatic SettingJ.A. Broomfield, H. Hirte, L. Elit, I. Kong, M. Patel, S. Voruganti,

C. Smith, T. Chow, and A. Swaminath; Juravinski Cancer Centre,

Hamilton, ON, Canada

Purpose/Objective(s): Stereotactic ablative radiation therapy (SABR) hasrecently emerged as an effective treatment option for cancer patients. It is

non-invasive, provides excellent rates of local control (LC) and is asso-

ciated with few serious side effects. In the metastatic setting, the popula-

tion of patients expected to receive maximum benefit from SABR is not

well defined, although those with oligometastatic disease (�3 involved

organs, �5 total metastases) are suspected. SABR data with respect to

gynaecological malignancies is particularly limited. Here we report one of

the largest single institution experiences on this subject.

Materials/Methods: We performed a retrospective review of all patients with

gynecological malignancies harboring oligometastatic disease treated with SABR

at our institution. Collected information included patient demographics, malig-

nancy characteristics and SABR details, as well as treatment outcomes including

LC, progression-free survival (PFS), overall survival (OS) and toxicities.

Results: Between March 2011 and November 2013, 21 patients with oligo-

metastatic disease were treated. Median age at time of stereotactic consultation

was 61 years (43-85). Nine patients had uterine, nine had ovarian and three had

cervical primaries. Adenocarcinomawas the most common histology (47.6%),

followed by papillary/serous (33.3%) and squamous cell (9.5%) variants. Tu-

mors were Grade 1, 2 and 3 in 19%, 19% and 42.9% of cases, respectively.

Median PFS following initial treatment(s) was 29.6 months (2.9-171.3). Two-

thirds of patients had a single metastasis treated with SABR (range Z 1-3),

located within nodal (55.2%), pulmonary (13.8%), intracranial (10.3%) and

hepatic (10.3%) tissues. Average lesion size was 3.8 cm (0.8-7.6) and median

prescription dose was 30 Gy over 5 fractions (ranging from 22-60 Gy/1-8

fractions).Median post-SABR follow-up timewas 8.5months (2.9-24.9)with a

LC rate of 92.6% (25/27 targets). Eight patients (38.1%) remain alive with no

evidence of disease progression. Of the 13 patients (61.9%) who did progress,

their median PFS was 11.1 months (1.5-36.6). Two patients (9.5%) received

additional SABR following progression, while 3 patients (14.3%) died from

theirmetastatic disease. Not a single serious (� grade 3) toxicitywas identified.

Conclusions: The use of SABR to treat oligometastatic disease in patients

with gynecological malignancies is safe and provides excellent LC. Pro-

gression outside of the treated area remains high and further research is

needed to conclude which patients may benefit most from this approach.

Author Disclosure: J.A. Broomfield: None. H. Hirte: None. L. Elit:

None. I. Kong: None. M. Patel: None. S. Voruganti: None. C. Smith:

None. T. Chow: None. A. Swaminath: None.

3772Comparison of Whole-Brain Integral Dose Between Whole-BrainRadiation Therapy and Stereotactic Radiosurgery of Multiple BrainMetastasesE. C. White, R. Wang, and J. Rahimian, Kaiser Permanente, Los Angeles,

CA

Purpose/Objective(s): To compare normal brain dose exposure between

whole brain radiation therapy and stereotactic radiosurgery (SRS) treat-

ments of multiple brain metastases.

Materials/Methods:We identified 5 patients who were treated for multiple

brain metastases in our department with a combination of whole brain and

stereotactic treatments. All patients received initial whole brain radiation

therapy and were then treated later for multiple brain metastases with SRS.

Whole brain radiation therapy was delivered with opposed lateral beams

and consisted of 35 Gy in 14 fractions in 3 patients, 30 Gy in 10 fractions

in 1 patient, and 30 Gy in 10 fractions followed by re-treatment of 20 Gy in

5 fractions in 1 patient. Linear accelerator based SRS was performed via

multi-isocenter techniques, using 5 dynamic conformal arcs or 4-5 cone-

based circular arcs per lesion. In general, 20 Gy was prescribed to the

lesion periphery. The whole brain tissue (minus the volume of SRS treated

lesions) was contoured. The integral dose exposure (Joules or Gy-Kg) to

the whole brain was then calculated for the both the whole brain and the

SRS treatment. This was divided by the volume of normal brain in Kg to

give units of Gy. All planning was done in radiosurgery planning with SRS

performed on the stereotactic radiosurgical linear accelerator.

Results: For the 5 patients, a total of 43 metastatic lesions were treated

with SRS (per patient, range 6-13). The average SRS individual lesion size

per patient ranged from 0.146 - 0.732 cm3. The sum SRS lesion size per

patient was 2.92 cm3, 3.04 cm3, 1.17 cm3, 1.64 cm3, and 5.86 cm3. The

whole brain integral dose from the whole brain vs SRS treatment per pa-

tient was: 65.02 vs 4.65 Gy-Kg (36.12 vs 2.58 Gy), 55.04 vs 3.16 Gy-Kg

(36 vs 2.07 Gy), 55.32 vs 2.06 Gy-Kg (35.9 vs 1.34 Gy), 49.68 vs 1.3 Gy-

Kg (30.63 vs 0.8 Gy), and 83.84 vs 3.65 Gy-Kg (51.56 vs 2.24 Gy). For the

5 patients, the average whole brain integral dose was 61.78 Gy-Kg (38.04

Gy) for the whole brain treatments vs 2.96 Gy-Kg (1.81 Gy) for the SRS

treatments.

Conclusions: SRS treatment of multiple brain metastases delivers signif-

icantly less integral dose to normal brain tissue than whole brain radiation

therapy. With multi-isocenter linear accelerator based treatments, multiple

small lesions can be treated to high dose while still sparing significant

normal brain tissue. This dosimetric advantage likely has clinical benefit

for late neurocognitive preservation. SRS of multiple metastatic lesions

should be considered over whole brain radiation therapy in patients where

minimizing integral brain dose and thus late neurocognitive deficit is

indicated.

Author Disclosure: E.C. White: None. J. Rahimian: None.

3773Development of a High-Resolution and High-Efficiency DosimetryStrategy for Robotic Radiosurgery QAB. Han, A. Ding, L. Xing, and L. Wang; Stanford University, Stanford, CA

Purpose/Objective(s): The increasing use of small field beams of

robotic radiosurgery system presents a significant challenge and calls for

new tools for dosimetric measurements. The purpose of this study is to

systematically investigate a high spatial-resolution (0.2mm) amorphous

silicon flat-panel electronic portal imaging device (EPID) for radiosurgery

quality assurance (QA).

Materials/Methods: The EPID-based dosimetric measurement technique

has been previous developed and validated using photon beams of a

medical linear accelerator. In this study, the same technique with machine

specific response and calibration is tested for the radiosurgery field output

measurement. To convert a raw EPID-measured image of a radiosurgery

cone field into water-based dose distribution, a pixel-to-pixel response of

the EPID specific to the robotic radiosurgery system is required. The

response function is obtained by using a Monte Carlo simulation of the

photon transport in the detector and a comprehensive calibration. After the

raw image is converted into the primary photon fluence, the fluence is

convolved into a water-based dose distribution of the radiosurgery cone

field by using a pre-generated pencil-beam kernel. The technique is applied

to measure output factors for radiosurgery fields formed by fixed cones and

the variable aperture collimator (Iris). 6MV Circular fields of 5, 7.5, 10, 15,

30 and 60mm diameter are measured and the results are compared with the