Abstract Book

ESTRO 37

S576

plan might be partly responsible, but cannot explain the larger dose reduction irrespective of smaller pV in BTH >12 group. With ADT for longer than a year, BT had a disadvantage compared to other irradiation modalities. PO-1026 LDR prostate brachytherapy inverse planning including dose-volume relation and tissue heterogeneity K.A. Mountris 1 , J. Bert 1 , D. Visvikis 1 1 INSERM UMR 1101 - LaTIM, Faculté de Médecine, Brest, France Purpose or Objective Current inverse planning systems for low-dose-rate (LDR) prostate brachytherapy employ meta-heuristic algorithms to predict implants’ position leading to the optimal dosimetric outcome. However, the optimality is compromised, mainly due to the dose overestimation induced by the AAPM TG-43 formalism, and limited control over the dose-volume relation. Our objective was to eliminate these issues, considering tissue heterogeneity during dose calculation using Monte Carlo (MC) dosimetry and evaluating the dose deposition to the volume of the prostate and the critical organs (urethra, rectum) in terms of dose-volume-histogram (DVH) evaluation during inverse planning. Material and Methods MC dose calculation is performed on a heterogeneous phantom of the patient’s anatomy including 4 materials (prostate, urethra, rectum, surrounding tissue) generated from the intraoperative ultrasound image. Needles crossing the prostate but no critical organs are selected and candidate implantation sites are extracted. MC dose kernels are pre-calculated prior to optimization for all the candidate sites. Dose calculations are performed using GGEMS, a graphics processing unit (GPU) MC simulation toolkit. The seed is modeled by a phase space, accounting for the exact seed geometric specifications and intra-seed particle interactions. The total number of seeds to be used is estimated by an empiric formula and an initial plan is generated by accumulating randomly selected seed MC dose kernels. The initial plan is optimized using fast simulated annealing (FSA) method. In each iteration of the FSA, the plan is updated by random single-seed swapping. The DVH of the updated plan’s dose map is calculated on the fly and the variation of the primary and secondary DVH metrics from the AAPM TG-137 report’s recommended values is minimized to obtain the optimal plan. Results For a database of 18 patients, the proposed inverse treatment planning method results in plans satisfying the TG-137 recommendations with 100% success. Compared to clinical plans delivered to the patients in this database, similar prostate V100 (0.2% difference) and reduced V150 by 6.1% are achieved. Furthermore, the urethra D10 and rectum D2cc are reduced by 4.0% and 0.6%, respectively. The implant optimization is performed in 30-45 s (15-20 s during MC dose kernel generation and 15 s during FSA). When tissue heterogeneity is not considered, prostate V100 is overestimated by 4%, similarly to previously published studies.

Conclusion The proposed method provides robust inverse planning with improved dose-volume response compared to standard clinical state-of-the-art treatment planning systems. The pre-calculation of MC dose kernels, using GPU, allows to consider tissue heterogeneity in intraoperative dose calculations. The implant’s optimization based on the DVH metrics minimizes the system’s learning curve for the user, since optimization and treatment quality criteria are identical. PO-1027 Dosimetric Improvement in HDR Prostate Brachytherapy Patients using Hydrogel Spacer Implantation S. Cavanaugh 1 , S. Crawford 2 , J. Dick 2 , P. Schantz 2 , T. Tsui 2 , K. Harpool 2 , W. Snyder 2 , J. Swanson 2 1 Cancer Treatment Centers of America, Radiation Oncology, Newnan GA, USA 2 Landauer Medical Physics, Radiation Oncology, Newnan GA, USA Purpose or Objective A commercially available bioabsorbable hydrogel system— a mixture of a precursor (trilysine buffer solution and polyethylene glycol powder) and accelerator (salt buffer)—is implanted between the prostate and rectum of men undergoing radiation therapy for prostate cancer. Use of this device provides additional separation between the prostate and rectum, ostensibly reducing rectal injury due to proximity of the high dose target. This study investigates rectal sparing achieved in high dose rate (HDR) prostate brachytherapy patients with hydrogel Two hundred and five monotherapy HDR prostate brachytherapy cases were selected from the patient population treated from July 2015 to October 10, 2017. Patients were selected based on a treatment regimen of 13.5 Gy per fraction. Of these cases, 173 fractions were delivered to patients with implanted hydrogel rectal spacers while 32 fractions were treated without the use of this or any other rectal sparing device. For this patient cohort, the dose volume histogram (DVH) from the previously calculated plan was used to obtain D 1cc , D 2cc , and maximum rectal dose statistics based on rectum contours drawn on the planning CT dataset by the dosimetrist at the time of treatment planning. To compare to conventionally fractionated external beam radiation therapy (EBRT), the equivalent dose in 2 Gy fractions (EQD2) was calculated for each rectal dose statistic. Statistical analysis software was employed for data analysis. Results The D 1cc and maximum dose to the rectum for cases in which the hydrogel spacer was implanted prior to treatment were compared to cases treated without rectal spacer implantation. Material and Methods