ESTRO 38 Abstract book

S1150 ESTRO 38

using Dice Similarity Co-efficient (DSC) and Hausdorff distances (Hf avg). Results Mean (±sd) volume of PTV, CTV of pCT was 60(±12)cc and 27(±7)cc respectively. Mean (±sd) volume of bladder and rectum of pCT vs CBCT averaged over 5 fractions were 368(±88) vs 345(±104)cc and 50(±14) vs 53(±14)cc respectively. Mean (±sd) DSC of bladder and rectum were 0.88(±0.7) and 0.92(±0.06), while Hfavg bladder and rectum were 2.6(1.4) mm and 1.2(1.0) mm respectively. Mean (±sd) difference in V 95% between the planned and the delivered dose for PTV was 4(±3)%. However, maximum variation in V 95% between the delivered and the planned dose was found to be correlating with the variation in bladder volume (r=0.42). Variation in other dose volume parameters was found to be negligible including for CTV. Mean (sd) difference between the planned vs delivered dose for bladder was 1.4 (5.7)%. However, patient wise analysis showed a maximum variation of 19% when the bladder volume variation was 138cc (patient-1). Mean (sd) difference between the planned vs delivered dose for rectum was 1.7 (±3.0)%, patient wise analysis showed a maximum variation of 4.0%, when the volume variation was 22 cc, and no correlation with rectal volume was observed. Conclusion The difference between the planned and the delivered dose to target volumes and OARs were quantified. Bladder volume variation was found to be correlating with dose to PTV, however, no correlation was found for rectum. EP-2084 Arms-down versus arms-up positioning for breast cancer patients receiving proton beam radiation. E. Batin 1 , N. Depauw 2 , S. MacDonald 2 , R. Jimenez 2 1 UMC Groningen Proton Therapy Center, Radiation Oncology, Groningen, The Netherlands ; 2 Massachusetts General Hospital, Radiation Oncology, Boston, USA Purpose or Objective Breast irradiation, with or without regional nodal involvement, is traditionally performed in an arms-up position. This is driven by the necessity to accommodate the photon treatment head while treating tangential fields in order to avoid collisions or treatment through the arms. This position can be uncomfortable for patients who, as a consequence of surgery, may have cording or contractures that limit arm mobility. Breast irradiation with proton therapy (PT) is performed using an enface approach, thus removing the treatment limitations of photons. This study evaluates the contouring, dosimetric, and practical challenges involved in performing PT breast irradiation in Ten breast patients underwent computed tomography (CT) simulation in both an arms-up and an arms-down position, following similar protocols for treatment. Contouring was performed on both arms up and arms down CT scans by two physicians with extensive proton breast treatment experience. Each set of contours was then verified by the non-contouring physician. Plans were generated with multi-criteria optimization and Pareto surface navigation using an in-house treatment planning system. Each treatment plan was generated using identical target coverage objectives, target constraints, and organs-at-risk (OAR) constraints, with each plan optimized by prioritizing target coverage within the confines of the a priori determined OAR constraints. No OAR objectives were used to avoid biasing the planning process. Two physicists was responsible for generating all plans. Each plan was then verified by the non-planning physicist. the arms-down position. Material and Methods

Conclusion The gamma test can be very sensitive to the noise of MC dose distributions. We propose here a method to correct this bias. The test remains fast (< 7 minutes including MC computations) and provides a PR way more reliable than the conventional gamma test. EP-2083 Evaluation of Deformable Image Registration and Dose Accumulation in Prostate SBRT Patients J. Swamidas 1 , R. Phurailatpam 1 , S. Panda 1 , V. Murthy 1 , K. Joshi 1 , D. Deshpande 2 1 ACTREC- Tata Memorial Centre, Radiation Oncology, Mumbai, India ; 2 Tata Memorial Hospital, Medical Physics, Mumbai, India Purpose or Objective To quantify the difference between planned and the delivered dose using deformable image registration (DIR) and deformable dose accumulation (DDA) for patients treated with Stereotactic Body Radiotherapy (SBRT) for prostate cancer. Material and Methods Ten prostate cancer patients previously treated with SBRT (35Gy in 5 fractions) were retrospectively analysed for this study. All patients were treated with Rapid Arc using 10MV FFF beams (Varian Eclipse TPS v13.5.37, True Beam v 2.1, Varian Medical Systems, Palo Alto). Daily cone beam CT(CBCT) was carried out as per the institutional protocol, which includes a bladder filling and a bowel protocol which reproduces the anatomy on the treatment day as compared to the planning day. Delivered dose was calculated by accumulating the doses using DIR and DDA in Velocity DIR software (v 3.2.1). Daily CBCTs were also imported and deformably registered to the planning CT (pCT) and generated a synthetic CT (sCT). RT plan was recalculated on sCTs. The dose calculated on sCTs were deformed back and accumulated onto pCT to estimate the total delivered dose-DDA. The planned and delivered doses were compared using various dose volume parameters for PTV, CTV and OARs (bladder and rectum). Target coverage was estimated by comparing the differences between the planned and accumulated doses of PTV and CTV in terms of V 95% (%), D mean (Gy) and D max (Gy). For OARs, D max (Gy), D mean (Gy),volume receiving various dose levels such as 30, 20, 10 and 5Gy were evaluated. Registration accuracy were also evaluated