ESTRO 2025 - Abstract Book

S1903

Clinical - Urology

ESTRO 2025

875

Digital Poster MR-guided hypofractionated salvage radiotherapy (MRgHFSRT) at 1.5 Tesla in post-prostatectomy patients: Preliminary clinical outcomes Darren MC Poon 1 , Jing Yuan 2 , Oi Lei Wong 2 , Bin Yang 3 , Sin Ting Chiu 4 , George Chiu 4 , Ben Yu 3 1 Comprehensive Oncology Centre, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong. 2 Research Department, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong. 3 Medical Physics Department, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong. 4 Department of Radiotherapy, Hong Kong Sanatorium and Hospital, Happy Valley, Hong Kong Purpose/Objective: Post-prostatectomy salvage radiotherapy (SRT) is a critical radical treatment for biochemical recurrence (BCR). Moderate hypofractionation offers similar efficacy and toxicity to conventionally fractionated regimens [1, 2]. However, the implementation of hypofractionated SRT is challenging because the prostate bed is a highly deformable target beside normal structures (e.g. the rectum). With several advantages, including enhanced soft tissue imaging and online adaptive planning, MRgHFSRT may improve treatment accuracy and safety [3]. This prospective study evaluated the preliminary outcomes of MRgHFSRT in post-prostatectomy patients. Material/Methods: We recruited consecutive patients who experienced BCR after prostatectomy for prostate cancer, with/without prostate bed/pelvic nodal recurrence detected on PSMA-PET. Patients with MRI contraindications, distant metastases, previous prostate/pelvic irradiation, or a history of non-prostate cancers were excluded. MRgHFSRT was delivered using a 1.5 Tesla MR-LINAC, with a daily 20-fraction scheme (prostate bed, 55 Gy; PSMA-avid sites in the prostate bed, 60 Gy; recurrent nodes, 55 – 60 Gy; pelvic lymphatics, 44 Gy). Androgen deprivation therapy was prescribed at the physician’s discretion. The planning target volume margins were overall 5– 8 mm (posterior, 5 mm) for the prostate bed and 5 mm for lymphatics. To maximize workflow efficiency, online plan adaptation was performed preferably with adapt-to-position (ATP, adjusting isocenter but no re-contouring), or adapt-to-shape (ATS, re-contouring on daily MRI) when center-customized criteria were violated. Patients were followed up per the institutional protocol. Adverse events (AEs) were assessed using CTCAE v5.0. PSMA-PET was arranged when post-RT BCR (prostate- specific antigen [PSA] ≥0.2 ng/mL) occurred or clinically indicated. Biochemical failure -free survival (bFFS) was the primary endpoint. Results: From April 2021 to April 2024, 38 patients received MRgHFSRT (Table). All fractions were delivered uneventfully (median duration: 43 minutes; range, 38 – 68 minutes), with 91% (693/760) using ATP and 9% (67/760) using ATS. The mean percentage volume change of the prostate bed was 3.3% (range: – 3.4% – 22.9%) at the first ATS, referring to the original plan. The median follow-up was 13.7 months (range, 0.7 – 40.6months). No patients experienced BCR (bFFS, 100%). At 12 months post-MRgHFSRT, one patient (1/26, 3.8%) had oligometastatic progression detected on PSMA-PET after experiencing PSA elevation but not BCR. Overall, one patient (1/38, 2.6%) had acute grade 2 diarrhea, and two (5.3%) had late (≥3 months) grade 2 AEs (rectal hemorrhage and urinary frequency, respectively). No grade ≥3 AEs were observed.