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Purpose: This study evaluated interfractional bladder volume variations and the resulting dosimetric changes during short-course radiotherapy (SCRT) 5 × 5 Gy (relative biological effectiveness) in patients with locally advanced rectal cancer.

Materials and Methods: Twenty patients received either protons or photons with daily cone-beam computed tomography (CBCT). All patients received the same drinking instructions prior to the planning computed tomography (CT) and each fraction. For each patient, volumetric modulated arc therapy (VMAT) and proton beam therapy (PBT) plans were generated. The bladder was delineated on each CBCT which were registered to the planning CT in the treatment planning system. The baseline bowel bag structure was adjusted accordingly for each bladder volume. Volumetric and dosimetric data for the bladder and bowel bag were then analyzed.

Results: Baseline bladder volumes were on average 71 cm3 larger than the average volume during treatment (95% confidence interval, 15 to 126). No significant difference was detected during treatment. Mean bladder doses decreased significantly from baseline to during treatment for both VMAT and PBT treatment plans (p = 0.021 and p = 0.002, paired two-sided t-test). Compared to baseline, the dose to the bowel bag adjusted for daily bladder volume increased by 3.8% for VMAT (t = –2.56, p = 0.019, two-tailed) and 18.7% for PBT (t= –2.415, p = 0.026, two-tailed).

Conclusion: We report consistently smaller bladder volumes during SCRT compared to baseline. This resulted in lower-than-expected mean bladder doses during the treatment course and consequently an increase in dose to bowel bag. Variations in bladder volume resulted in larger changes in delivered dose to bladder and bowel bag in PBT compared to VMAT.