ESTRO 2025 - Abstract Book

S1983

Clinical - Urology

ESTRO 2025

2131

Digital Poster Factors associated with site of first failure on PSMA PET/CT imaging following adjuvant or salvage radiotherapy after radical prostatectomy Nicholas Dietrich 1,2 , Xiang Y Ye 3 , Alejandro Berlin 1,4 , Charles Catton 1,4 , Peter Chung 1,4 , Rachel Glicksman 1,4 , Enrique Gutierrez 1,4 , Andrew J McPartlin 1,4 1 Radiation Medicine Program, Princess Margaret Cancer Centre, Toronto, Canada. 2 Temerty Faculty of Medicine, University of Toronto, Toronto, Canada. 3 Department of Biostatistics, Princess Margaret Cancer Centre, Toronto, Canada. 4 Department of Radiation Oncology, University of Toronto, Toronto, Canada Purpose/Objective: Randomized trials have failed to demonstrate a benefit from dose escalation to the prostate bed during adjuvant/salvage radiotherapy (sRT). However, possible local control benefit for some subgroups is inconsistently suggested. Biomarkers identifying patients most at risk of local failure (LF) following sRT may facilitate personalisation of care. We investigated whether patient, clinical, and treatment factors at the time of sRT may predict for subsequent PSMA-identified LF. Material/Methods: Consecutive patients from a prospective multicenter imaging registry who had a positive 18 F-DCFPyL PSMA PET/CT scan between 2018 and 2024 following biochemical failure after sRT post-radical prostatectomy (RP) were considered. Those with local first site of failure (with or without regional/distant failure) were identified. Univariable (UVA) and multivariable (MVA) logistic regression analyses were performed to identify clinical, pathological, and treatment-related factors associated with development of LF on PSMA imaging. Results: 302 eligible patients were included. The median age at RP was 62.3 years (IQR: 58.1 – 66.5), median interval to sRT was 364 days (IQR: 163-1,057), and the median PSA level at the time of sRT was 0.23 ng/mL (IQR: 0.12-0.47). Following sRT, the median time to PSMA imaging was 1,980 days (IQR: 1,201-3,247), with a median PSA level of 0.86 ng/mL (IQR: 0.44-2.48). A total of 71 patients (23.5%) experienced PSMA LF as the first failure. In the minority with confirmatory MRI and/or biopsy, the positive predictive value (PPV) of prostate bed uptake was 81.3%. On UVA (Table 1), Gleason grade groups 1 (OR: 8.75, p =0.006) and 3 (OR: 3.41, p =0.031) vs grade group 5, and pN0 status (OR: 4.95, p =0.030) were associated with higher likelihood of LF. However, grade group 1 patients had a significantly longer interval to PSMA imaging than higher grade disease (median 4,015 vs 1,589 days, p =0.004). On MVA, Gleason grade groups 1 and 3 were the only factors significantly associated with LF.