1001 - Post-Prostatectomy Linac-Based Ultrahypofractionated Radiotherapy for Patients with Localized Prostate Cancer: Toxicity and Quality-of Life Results from a Prospective Trial

C. L. Tseng, D. Vesprini, M. T. M. Davidson, S. K. Liu, J. Detsky, P. Cheung, W. Chu, H. T. Chung, G. Morton, E. Szumacher, V. Escueta, A. Mamedov, L. Zhang, and D. A. Loblaw; Department of Radiation Oncology, Odette Cancer Centre, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, ON, Canada

Purpose/Objective(s): Ultrahypofractionated or stereotactic body radiotherapy (SBRT) is an established treatment option in intact prostate cancer, however, limited data exists on outcomes in the post-prostatectomy patient. We report toxicity and health-related quality of life (HRQOL) outcomes with SBRT in the post-prostatectomy setting in a single institutional prospective trial. Materials/

Methods: Post-prostatectomy men with histologically confirmed prostate adenocarcinoma were eligible if they had pathologic stage T3/T4, NX-0, M0 disease without gross residual, and/or a rising serum prostate specific-antigen (PSA) on at least 2 consecutive measurements. Treatment consisted of 30 Gy to the prostate bed, with optional elective nodal irradiation (ENI) to a dose of 25 Gy in 5 weekly fractions. Up to 24 months of androgen deprivation therapy (ADT) was permitted with SBRT. The primary endpoint was acute (= 3 months) genitourinary (GU) and gastrointestinal (GI) toxicities based on CTCAE v5.0. Secondary endpoints included late (= 6 months) GU and GI toxicities, as well as acute and late HRQOL assessed using the Expanded Prostate Cancer Index Composite (EPIC) questionnaire. Mild, moderate, and severe minimal clinically important change (MCIC) in HRQOL was defined as an EPIC score decrease of > 0.5, > 1.0, and > 2.0 standard deviation, respectively, from the baseline for each domain.
Results: Thirty patients (median age 64) were enrolled between November 2019 and March 2021. Median follow-up was 37.7 months (range, 35.0 – 42.6 months). Median PSA prior to radiation was 0.20 (IQR 0.10 – 0.38). Only 1 patient was treated adjuvantly and the remainder was treated with salvage intent. Thirteen (43.3%), 7 (23.3%), and 10 (33.3%) patients had pT2, pT3a, and pT3b disease, respectively, where 14 (46.7%) had a positive margin. Sixteen (53.3%), 8 (26.7%), 4 (13.3%), and 2 (6.7%) patients had ISUP Grade Group 2, 3, 4, and 5 disease, respectively. A total of 12 (40.0%) patients received ADT for a median duration of 6.5 months (range, 2.9 – 24.4 months) and 16 (53.3%) received ENI. There were no grade = 3 acute toxicities. The cumulative incidence of worst acute grade 2 GU & GI toxicities were 3.3% and 30.0%, respectively. Four late grade 3 toxicities were observed (1 urinary incontinence, 3 erectile dysfunction). Eight (26.7%), 16 (53.3%), and 3 (10.0%) patients reported acute moderate/severe MCIC in urinary, bowel, and sexual domains, respectively, while 2 (6.7%), 3 (10.0%), and 3 (10.0%) patients reported late moderate/severe MCIC in urinary, bowel, and sexual domains, respectively.
Conclusion: Post-prostatectomy SBRT to the prostate bed, with or without ENI, is well tolerated with no grade = 3 acute toxicities and minimal impact on late HRQOL. Further follow-up is warranted to better evaluate long-term toxicity and biochemical disease-free survival.