LBA 20 - Randomized Phase II Trial of Stereotactic Body (SBRT) vs. Conventional Fractionation (CF) Radiotherapy Boost for High-Risk Prostate Cancer: Biochemical Control Outcomes of the PBS Trial
Presenter(s)
L. D. Ben-Zvi1, F. Honarvar1, G. Dhaliwal2, G. Carr3, A. G. Gouveia4, A. Mesci5, I. Dayes1, H. Lukka1, K. Quan1, M. Goldberg1, K. Schnarr1, A. Hallock6, D. Cuthbert6, T. Chow1, and T. Tsakiridis1; 1McMaster University, Juravinski Cancer Centre, Hamilton, ON, Canada, 2TMU School of Medicine, Brampton, ON, Canada, 3Michael G. DeGroote School of Medicine, McMaster University, Hamilton, ON, Canada, 4Prince George Site, BCCA, Prince George, BC, Canada, 5Princess Margaret Hospital, UNH, University of Toronto, Toronto, ON, Canada, 6Walker Family Cancer Centre, Niagara Health System, St. Catharines, ON, Canada
Purpose/Objective(s): SBRT prostate boost treatment may improve disease control while maintaining acceptable toxicity and shortening treatment duration, but remains investigational. PBS is a randomized phase II trial that evaluates prostate SBRT boost in patients with unfavorable-intermediate or high-risk localized prostate cancer at two institutions. Quality-of-life and toxicity outcomes of this trial were reported earlier. This analysis examines biochemical failure-free survival (bFFS) amongst patients treated at two participating centers.
Materials/Methods: Eligible patients were accrued from 2019 to 2022 and randomized to pelvic external beam radiotherapy (EBRT; 45–46 Gy in 23–25 fractions) followed by either daily conventional fractionation (CF) prostate boost (34–35 Gy in 15–17 daily fractions) or weekly SBRT boost (19.5–21 Gy in 3 weekly fractions). All patients received androgen deprivation therapy (ADT), planned for a total duration of three years, using 6-month LHRH agonist injections. Biochemical failure was defined by the Phoenix criteria. Eugonadal testosterone recovery was defined as 8.0 nmol/L. Biochemical recurrence, distant metastasis, and survival outcomes were compared using Firth-penalized Cox proportional hazards models.
Results: Of the 72 patients included in this analysis, 38 received CF and 34 received SBRT. The majority had high-risk disease (88%), and the remaining had unfavorable-intermediate risk. After a mean follow-up of 4.3 years, 33% of patients had achieved eugonadal testosterone recovery. There were no significant differences observed between recurrence and survival outcomes in the two treatment groups. There were 2 biochemical recurrences in the CF arm and 1 in the SBRT arm. The bFFS at 4 years was 97.4% for CF and 98.2% for SBRT (HR = 0.68, 95% CI [0.06, 5.11], p = 0.7). There was one case of distant metastasis in each treatment arm and distant metastasis free survival was 96% in both groups (HR = 1.04, 95% CI [0.08, 12.8], p = 0.98). Seven deaths occurred, of which one was related to prostate cancer. The overall survival was 94.3% in the CF arm and 97.1% in the SBRT arm (HR 0.51, 95% CI [0.09, 2.10] p = 0.36).
Conclusion: This early analysis of the PBS trial suggests that pelvic EBRT followed by SBRT prostate boost demonstrates excellent biochemical control and survival in combination with ADT, similar to CF prostate boost. This analysis is limited by short length of follow-up and low hormonal recovery events after ADT completion. Longer-term follow-up is required to reliably assess eugonadal and overall survival outcomes.