IUC25511-7 MERIT AWARD: Prostate volume changes during longer fractionated and ultra-hypofractionated MR-guided adaptive radiotherapy leveraging the MOMENTUM registry

Background Prostate volume changes during radiotherapy may compromise target coverage. We quantified longitudinal swelling across longer fractionation (≥15 fractions)(LFRT) and stereotactic body radiotherapy (SBRT) regimes during MR-guided adaptive radiotherapy (MRgART) to identify predictors of clinically significant swelling (CSS, defined as ≥ 15% volume increase). Methods This multicentre prospective cohort study analysed patients treated on an MR-Linac between January 2020 and April 2025, enrolled in MOMENTUM [6]. Inclusion mandated daily recontouring/adaptation with ≥3 fractions available. Structure volumes were extracted using PyRadiomics (v3.0.1). A hierarchical fuzzy-matching algorithm (Levenshtein distance) harmonised prostate volume nomenclature across centres. Clinical and demographic variables were collected, and fraction 1 was counted as the baseline volume [7]. The proportion of treatment spent with CSS was normalised to the number of fractions. Baseline prostate volume was dichotomised. Linear mixed-effects models (LMM) quantified continuous volume trajectories by cohort. Multivariable logistic regression tested predictors of CSS as a binary outcome. Results From 1907 patients, 222 LFRT and 1390 SBRT patients met eligibility. A total of 10297 fractions were analysed (median 20 for LFRT, 5 for SBRT patients), with median baseline prostate volume of 40.0 cm³ (IQR 28.9-53.6) for LFRT and 47.5 cm³ (IQR 35.8-66.0) for SBRT. Incomplete age data excluded 160 patients from LMM. CSS occurred in 15.2% LFRT and 13.1% SBRT total fractions (Figure 1), but SBRT patients spent a higher proportion of treatment in CSS (Figure 2). In LMM, larger baseline prostate volume was associated with reduced mean swelling in SBRT (β = -0.030% per cm³, p < 0.001) but not LFRT (p = 0.30). However, logistic regression showed larger baseline volume was protective in both cohorts (LFRT: OR 0.38, 95% CI 0.32-0.46; SBRT: OR 0.80, 95% CI 0.69-0.92). In secondary complete-case analysis including ADT and T-stage data (n = 116 LFRT, n = 256 SBRT), no significant association with ADT was observed in either cohort (LFRT p = 0.65; SBRT p = 0.37), whilst T2-stage disease was associated with increased swelling in SBRT (β = +3.0%, p = 0.018) compared to T1-stage. Conclusions SBRT patients experience a faster onset of prostate swelling compared to LFRT. Smaller baseline volumes and T2-stage were associated with increased swelling risk. Incomplete clinical covariate data limits definitive risk stratification, supporting continued fraction-by-fraction adaptive workflows.