Target-Dose Robustness During Major Pelvic Deformation After HDR Prostate Brachytherapy: Fraction-Level and Accumulated-Dose Effects of Online Adaptive Radiotherapy

Background: Major pelvic deformation after HDR prostate brachytherapy can compromise fraction-level radiotherapy coverage. We compared scheduled and online-adapted dose distributions in a case complicated by a large pelvic hematoma. Methods: One 13.5-Gy HDR fraction was followed by 52.5 Gy EBRT in 21 fractions (19 adaptive Ethos and two non-adaptive TrueBeam fractions). Nineteen paired daily scheduled/adapted plans were compared. Complete EBRT physical-dose accumulation was independently repeated in MIM 7.3.7 for four combinations (Adapted/Scheduled × deformable/rigid registration); 19 image pairs underwent quantitative geometric registration QA, and accumulated DVHs were exported from Eclipse. Results: Scheduled PTV D95 missed the 98% objective in 17/19 sessions (median 90.3%) versus 1/19 after adaptation (median 98.4%; median paired improvement +8.0 percentage points). PTV D99 medians were 73.7% and 95.7%, respectively. With deformable accumulation, PTV D95 was 47.928 Gy (Scheduled) versus 49.044 Gy (Adapted), whereas PTV V52.5 Gy was 67.4% versus 57.9%; with rigid accumulation, D95 was 48.666 versus 48.761 Gy. Patient-specific contour agreement improved with DIR for the bladder and hematoma but not consistently for CTV/PTV, while rectal agreement remained low. Conclusions: Online adaptation restored fraction-level PTV coverage; cumulative target and OAR indices showed endpoint-dependent trade-offs and registration-method sensitivity. Deformable dose accumulation remains an exploratory estimate rather than validated tissue-specific delivered dose, and no clinical outcome advantage can be inferred.