2015 - Two Novel Patient-Specific Strategies to Account for Intrafractional Bladder Filling during Online Adaptive Radiotherapy for Bladder Cancer Incorporating a Focal Boost
Presenter(s)
S. Azzarouali1,2, K. Goudschaal2,3, J. Visser2,3, L. Daniels2,3, A. Bel2,3, and D. den Boer1,2; 1Radiation Oncology, Amsterdam UMC location Vrije Universiteit Amsterdam, De Boelelaan 1117, Amsterdam, Netherlands, 2Cancer Center Amsterdam, Cancer Therapy, Treatment and quality of life, Amsterdam, Netherlands, 3Radiation Oncology, Amsterdam UMC location University of Amsterdam, Meibergdreef 9, Amsterdam, Netherlands
Purpose/Objective(s): Variable bladder filling challenges radiotherapy (RT) for bladder cancer between and during treatment fractions. Online adaptive RT (oART) reduces interfraction inaccuracies by acquiring a planning CBCT, adapting the treatment plan and patient positioning during sessions. RT delivery follows, with a post-CBCT acquired. To give a focal boost, patient positioning prioritizes GTV coverage, potentially at the expense of bladder coverage. Current practice uses population-based margins to estimate intrafraction bladder filling, generally not accounting for this couch shift. This study aims to evaluate two strategies to account for patient-specific intrafractional bladder motion during oART in bladder cancer incorporating a focal boost. The hypothesis is that the proposed patient-specific strategies enhance treatment precision by minimizing healthy tissue volume receiving high dose compared to a population-based strategy.
Materials/Methods: To account for bladder filling during treatment, margins were applied to 100 planning CBCTs and evaluated on 100 post-treatment CBCTs from five bladder cancer patients treated with oART (40 Gy + 15 Gy boost in 20 fractions using fiducial markers). For strategy 1, two CTs were acquired per patient at a 15 min interval to estimate bladder filling. The bladder was delineated on both CTs with a shift applied prioritizing GTV coverage. Margins to cover the post-bladder were determined by expanding the pre-bladder in six directions, selecting the margins leading to the smallest volume. To account for longer session times (22 min), margins were extrapolated from those determined without a couch shift. The online pre-bladders were expanded by these margins and analyzed for coverage of the post-bladder and healthy tissue inclusion (inside PTV), compared to population-based margins (11 mm, 17 mm superior). For strategy 2, margins were determined by delineating pre-bladder contours on planning CBCTs for session 2-5 (excluding session 1 due to chemotherapy effects). A couch shift was applied for optimal GTV alignment and margins were determined on post-CBCTs. The average margin was applied to subsequent sessions, with the same analysis performed as for strategy 1.
Results: With strategy 1 and 2, the median [min-max] volume covered by the post-bladder was 98% [73-100] and 100% [86-100], respectively, compared to 100% [76-100] for population-based margins while healthy tissue volume inside PTV was 217 [38-523] , 136 [84-246] and 190 [78-418] cm3, respectively. A statistically significant reduction in healthy tissue inclusion was observed with strategy 2 compared to population-based (Wilcoxon signed-rank test, p = 0.05).
Conclusion: This study suggests that incorporating a patient-specific strategy (strategy 2) in oART for bladder cancer including a focal boost may enhance treatment precision by reducing healthy tissue volume inside PTV. A follow-up study with a larger patient cohort should confirm these findings and inform clinical guidelines.