S586 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008

Conclusions: ROI analysis is an elegant and efficient tool to explore and evaluate the optimal strategy of an adaptive IMRT. Treat-ment delay from planning was found an important factor to improve target coverage and OAR sparing for whole pelvic IMRT forcervix cancer patients.

Author Disclosure: Y. Cho, None; J. Xie, None; V. Kelly, None; K. Lim, None; J. Stewart, None; A. Fyles, None; K. Brock, None;A. Lundin, None; H. Rehbinder, None; M. Milosevic, None.

2930 Inverse Planning Simulated Annealing Optimization in CT-based Intracavitary HDR Brachytherapy for

Gynecological Cervical Cancer

Z. Wang, H. K. Malhotra, W. Jaggernauth, M. B. Podgorsak

Roswell Park Cancer Institute, Buffalo, NY

Purpose/Objective(s): CT or MRI based intracavitary brachytherapy has emerged as a useful clinical modality in the treatmentof gynecological cervical cancer. Conventional dose optimization in HDR brachytherapy is based on forward planning (FP)approaches. Recently, an inverse planning simulated annealing (IPSA) algorithm has been introduced to achieve optimized3D dose distribution. This study is to evaluate the use of IPSA versus FP approaches in tandem and ovoid intracavitary brachy-therapy.

Materials/Methods: We retrospectively studied 12 HDR tandem and ovoid treatments. In all cases, the target volume drawing andthe subsequent plan approval were carried out by the same radiation oncologist. The CT-based treatment plans were generatedusing Nucletron PLATO Brachytherapy planning system. We used FP approaches for the clinically treated plans, which involveddose point optimization, graphic optimization and dose volume histogram evaluation between each step of manual tweaking priorto reaching an acceptable plan. The prescribed dose was based on D90 to the target and the mean D90 of the 12 plans was 530 cGy.The planning goal was to limit the fractional rectal dose to 421 cGy to 0.1 cc, 380 cGy to 1 cc and 350 cGy to 2 cc of rectum.1 Thecorresponding fractional dose limit to bladder was D0.1cc = 700 cGy, D1cc = 620 cGy and D2cc = 580 cGy.1 (1Radiother. Oncol.78:67-77.) In IPSA planning, we set up a standard IPSA dose constraint table for all the 12 plans. The IPSA generated planswere normalized to the original treated D90. We also calculated the D0.1cc, D1cc and D2cc to the rectum as well as the bladder,and the V100, V150 and V200 of the target. The Conformity Index (CI) = (prescribed dose volume)/(target volume) and theDose Homogeneity Index (DHI) = [1 - (V150/V100)] were then calculated to compare the IPSA plans with the FP plans.

Results: The mean V100 of the target is 90.1% in IPSA plans and 90.0% in FP plans, while the mean V200 is 34.7% in IPSA vs.37.3% in FP. IPSA plans show slightly better CI, which is 1.57 ± 0.1 in IPSA vs. 1.63 ± 0.2 in FP. The DHIs are comparable, whereDHI is 0.37 ± 0.04 in IPSA vs. 0.35 ± 0.05 in FP. Dose volume analysis of the fractional rectum dose shows the mean values ofD0.1cc = 376 cGy, D1cc = 330 cGy and D2cc = 309 cGy in IPSA vs. the corresponding doses of 413, 359, and 333 cGy in FP. Similarbetter results of IPSA also show in bladder dose, which is 471, 374, and 365 cGy in IPSA vs. 484, 408, 372 cGy in FP, for 0.1 cc, 1cc and 2 cc, respectively.

Conclusions: IPSA is a useful optimization tool in 3D imaging based intracavitary HDR brachytherapy, which has demonstratedadvantages over the conventional FP approaches. With IPSA, further improvement may still be required by manually fine adjustingthe isodose distribution, while the amount of time to finalize an acceptable plan will be substantially reduced.

Author Disclosure: Z. Wang, None; H.K. Malhotra, None; W. Jaggernauth, None; M.B. Podgorsak, None.

2931 Dosimetric Consequences of the Prescription Point H of ABS Recommendation in the Era of MRI Guided

Brachytherapy for Cervical Cancer: Based on GYN GEC-ESTRO Recommendations of MRI GuidedBrachytherapy

Y. Kim, Y. Huang, J. E. Bayouth, R. T. Flynn, S. K. Bhatia, G. M. Jacobson, J. M. Modrick

University of Iowa, Iowa City, IA

Purpose/Objective(s): The GEC-ESTRO recommendations for MRI guided brachytherapy (BT) have strictly recommended notemploying a solely volume-based prescription (Rx) before obtaining sufficient clinical data. Even in the era of MRI guided BT, theRx point is still critical, incorporating MRI information in a hybrid way such as that the dose to the target volumes is increased to theRx point dose. Hence, the dosimetric consequences of prescribing the radiation dose to different points were determined.

Materials/Methods: Three different Rx points were explored: a revised Manchester Point A (Point A), the Point A utilized at theUniversity of Iowa (Point A-UI: superiorly move from the tip of ovoids), and the Point H as proposed by the ABS. For stage I, II,and III cervical cancer, thirty high dose rate patients’ plans prescribed to Point H were randomly selected. To remove the variationsfrom anatomy changes through a treatment course, only the first fraction plans were retrospectively renormalized according to eachPoint A and Point A-UI. No variations were made other than the modification of the Rx points. A tandem and ovoid Fletcher SuitDelclos style applicator set (Varian) was utilized for all patients. Based on the GEC-ESTRO recommendations and ICRU Report38, the values of total reference air kerma (TRAK) and the volume receiving 100% Rx dose (VOL100%Rx), both representing thedelivered amount of radiation, were determined and the ICRU rectal and bladder point doses were quantified.

Results: Prescribing at Point H resulted in the minimal variations in a given geometry and using Point A-UI showed smaller var-iations than prescribing at Point A. The percent deviations against Rx doses were up to ±9.5% (mean ±2.2%, std dev ±3.0%, p =0.0001) for Point A, ±8.6% (mean ±2.1%, std dev ±2.4%, p\0.0001) for Point A-UI, and ±6.7% (mean ±1.8%, std dev ±1.3%:p \ 0.0001) for Point H. When normalized to the Point H values, the percent (absolute) differences in TRAK were found onaverage -2% (-110 cGy$cm2) and -1% (-65 cGy$cm2) for Point A and A-UI, respectively. The percent differences in VOL100%Rx

were on average -3% (std dev 5.8%) and -2% (std dev 2.9%) for Point A and A-UI, respectively. For both the rectum and the bladderpoints the percent differences were determined as on average -2% for Point A and -1% for Point A-UI. Prescribing at Point Hincreased the total delivered radiation dose since Point H was superiorly located.

Conclusions: A brachytherapy plan is minimally affected from geometrical variations when a radiation oncologist prescribes doseto Point H. Prescribing at Point H increased 1�2% of total amount of radiation but its clinical impacts need further studies.