S2140
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
10.3389/fonc.2022.916254. Keywords: MRgRT, synthetic CT, SBRT
Digital Poster 2531
Bladder dose accuracy in CBCT-guided prostate radiotherapy: How important is bladder filling consistency? Meriem Skouri, Zeineb Naimi, Raouia Ben Amor, Awatef Hamdoun, Ghada Bouguerra, Rihab Haddad, Lotfi Kochbati Radiation Oncology Department, Abderrahmen Mami Hospital, Ariana, Tunisia Purpose/Objective: The aim of this study was to assess the impact of daily bladder volume variability on delivered bladder doses in CBCT-guided VMAT for prostate cancer. Material/Methods: Forty patients with non-metastatic prostate cancer were prospectively enrolled in this study. All patients underwent standard fractionation VMAT with a prescription dose of 76Gy in 38 fractions. Daily CBCTs were performed for image guidance. Bladders were contoured on each CBCT, and the initial treatment plan was recalculated on each scan. Bladder volumes and dosimetric parameters (V50Gy, V60Gy, V65Gy, V70Gy, V75Gy and Dmax) were collected and statistically compared to baseline values. Pearson correlation and linear regression analysis were performed to assess the dosimetric impact of bladder volume variability. Results: Data from 1560 scans was analyzed. A significant reduction in bladder volume was observed during treatment (p<0.001), down to 69% of the baseline volume, with a mean relative variability of -30.9 % ranging from -59% to +10%. In parallel, bladder dose parameters significantly increased (p<0.001). Mean relative variabilities of V50Gy, V60Gy, V65Gy, V70Gy and V75Gy were +40.6%, +60.7%, +69.7%, +81.2% and +105.5%, respectively. All bladder dose constraints were met, except for Dmax, which was exceeded in one patient. Relative bladder volume variability was strongly correlated with relative dosimetric variability (r= -0.6 to -0.5) (p<0.001). Bladder doses increases correlated linearly with bladder volume reductions, with low to moderate predictive values (R2= 0.25 – 0.44 ; p<0.001).
Conclusion: Daily bladder volume changes significantly affect bladder dose distribution in prostate radiotherapy. Maintaining a stable bladder volume throughout treatment is essential to ensure treatment plan accuracy and optimal dose delivery. Keywords: Prostate cancer, CBCT, Bladder volume
Poster Discussion 2700
Clinical impact of adaptive proton therapy in esophageal cancer: improvements in target robustness and cardiac sparing Vlad Badiu, Esther Kneepkens, Dominique Reijtenbagh, Maaike Berbée, Richard Canters Department of Radiation Oncology (Maastro), GROW School for Oncology and Reproduction, Maastricht University Medical Centre+, Maastricht, Netherlands Purpose/Objective: The sharp distal dose gradients characteristic of intensity-modulated proton therapy (IMPT), combined with inter-fractional anatomical changes, can lead to target underdosage or unintended dose delivered to organs of interest, particularly the heart. Adaptive proton therapy compensates for anatomical variations through replanning. This study evaluates the clinical impact of adaptation in esophageal cancer, focusing on dosimetric differences and predicted mortality risk between adaptive and non-adaptive treatment workflows. Material/Methods: A retrospective analysis was performed on 192 esophageal cancer patients treated with IMPT (85 adapted, 107 treated non-adaptively). Adaptation was triggered by target underdosage (ITV D98% <94% of prescription), heart overdose ( ≥ 1.5 Gy increase in MHD), or both. Robust, weighted dose accumulation was performed by mapping all fraction doses – individually assigned to the nearest repeated weekly CT – to the planning CT, incorporating ±2mm residual setup variations and ±3% density perturbations to generate voxel-wise minimum (VWmin) and maximum (VWmax) dose distributions. Two accumulation strategies were compared:Multiple Plan Accumulation (MPA): included all adaptive plans.Single Plan
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