ESTRO 36 Abstract Book

S242 ESTRO 36 2017 _______________________________________________________________________________________________

2 Sheffield Hallam University, Faculty of Health & Well- being, Sheffield, United Kingdom Purpose or Objective Due to the significant variation of bladder volume observed throughout the course of treatment, various adaptive strategies have been developed to improve the quality of bladder radiotherapy. The aim of this study was to use deformable registration and dose accumulation processes to compare the dosimetric differences of a population-based PTV approach and three proposed adaptive strategies: Plan of the Day (POD), Patient- Specific PTV (PS-PTV) and daily reoptimization (ReOpt). Material and Methods Bladder patients (n=10) were included in this retrospective investigation. Patients were planned and treated with a full bladder in supine position. Planning CT and the CBCTs were retrieved and imported into treatment planning system. After delineating the bladder and the pelvic lymph node (PLN) on the planning CT, an expansion of 1.5 cm and 0.5 cm was applied to generate the population-based Standard PTV WB and PTV PLN , respectively. A 7-field IMRT distribution was designed to deliver a prescription dose of 46Gy in 23 fractions. Each adaptive strategy was applied according to published guidelines. After simulating the execution of each strategy using the daily CBCTs, daily dose was computed on all CBCTs and then total dose was summed on the planning CT using the output from the CT-CBCT deformable image registration. The volume receiving 95% of prescription dose (V 95 ) was compared against the Standard for each of the adaptive strategies. p < 0.05 was considered statistically significant. Results Mean V 95 (cm 3 ) were 1410 (SD: 227), 1212 (SD: 186), 1236 (SD: 199), and 1101 (SD: 180) for Standard, POD, PS-PTV and ReOpt, respectively. All adaptive strategies significantly reduced the irradiated volume, with ReOpt demonstrating the greatest reduction compared to the Standard (-25%). This was followed by a mean reduction of 16% with PS-PTV and 12% with POD. The difference in the magnitude of reduction between ReOpt and the other 2 strategies reached statistical significance (p = 0.0006). Conclusion Previous comparisons between bladder adaptive strategies have been limited due to the inability to account for the effect of daily motion of the bladder and surrounding organs. When deformable registration is used to reconstruct dose in the presence of organ motion, ReOpt is the best adaptive strategy at reducing the irradiated volume due to its frequent adaptation based on the daily geometry of the bladder. However the resource burden associated with this strategy needs to be quantified to further assess the feasibility of clinical implementation. PV-0461 Integrating diagnostic MRI in radical bladder cancer radiotherapy: Challenges in image registration. C.L. Eccles 1 , H. McNair 1 , D. McQuaid 2 , K. Warren-Oseni 2 , V.N. Hansen 2 , A. Sohaib 3 , M.D. Koh 4 , R. Huddart 4 , S. Hafeez 4 1 The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, Radiotherapy, London, United Kingdom 2 The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, Radiotherapy Physics, London, United Kingdom 3 The Royal Marsden NHS Trust, Radiology, London, United Kingdom 4 The Royal Marsden NHS Foundation Trust and The Institute of Cancer Research, Radiotherapy and Imaging, London, United Kingdom Purpose or Objective Radiographer led soft tissue matching has been fundamental for implementation of adaptive strategies in

Conclusion The analysis of the potential failures, their causes and effects allowed us to increase the quality and the safety in the CK workflow process. The FMECA technique provides a systematic method to target vulnerabilities before they generate an error. This framework analysis can naturally incorporate further quantification and monitoring. The FMECA method is an effective tool for the management of risks in patient care. PV-0459 Prostate CBCT dose optimization : from an iterative mAs reduction to a sytematic exposure reduction E. Jaegle 1 , M.E. Alayrach 1 , A. Badey 1 , V. Bodez 1 , C. Khamphan 1 , P. Martinez 1 , R. Garcia 1 1 Institut Sainte Catherine, Physique, Avig non, France Purpose or Objective A daily repositioning Cone Beam Computed Tomography image (CBCT) for prostate radiotherapy is realized using exposure templates (mAs, kV) which affect image quality and imaging dose. Settings should be optimized to minimize patient exposure while maintaining sufficient image quality to register the initial planning CT with CBCT using soft tissue matching. Material and Methods 20 prostate patients (without hip prosthesis) with daily CBCT (40 fractions) acquired on a TrueBeam™ (Varian Medical Systems) machine were selected. After the first fraction using the standard pelvis template (125 kV 1080 mAs CTDIw 14 mGy), the therapists manually applied, day after day, a low mAs reduction and assessed if the CBCT image quality was good enough for patient repositioning. The iterative process stopped when image quality was assessed too bad and the last proper mAs were selected. The link between the mAs reduction and corpulence (patient volume inside CBCT FOV) was studied. For one example patient, 23 therapists registered CBCT images with CT for 3 fractions : the first fraction (S 0% ), a fraction with 50% mAs reduction (S -50% ) and the fraction with maximum mAs reduction (S -71% ). Fisher’s test was applied to every direction, to compare the variance between S 0% / S -50% and S 0% / S -71% . Results The median mAs reduction for all patients was 64% (13% to 85%). Patient corpulence was not correlated to the mAs reduction achieved (Spearman’s correlation r s = 0.465). Variance analysis, for every direction, shows no significant difference (p<0.05) between S 0% / S -50% and S 0% / S -71% . Table 1 : For 3 fractions, the variance in matching from 23 therapists and Fisher’s test results Conclusion mAs reductions recorded across the 20 patients are highly variable, due to the subjective assessment of CBCT image quality, but a median reduction of 64% indicates a great potential for reducing imaging dose. For one patient it has been demonstrated that image quality deterioration has no impact on interobserver variability. A 50% mAs reduction for the Pelvis CBCT template is therefore considered. PV-0460 Comparison of 3 Image-guided Adaptive Strategies for Bladder Radiotherapy V. Kong 1 , A. Taylor 2 , T. Craig 1 , P. Chung 1 , T. Rosewall 1 1 Princess Margaret Cancer Centre, Radiation Medicine Program, Toronto, Canada