Salvage Radiotherapy for Recurrent Prostate Cancer: Can the Prognostic Grade Group System Inform Treatment Timing?

Purpose: In order to better time salvage radiotherapy (SRT) for post–radical prostatectomy biochemical failure, we examined the association between pre-SRT prostate-specific antigen (PSA) and PSA control as a function of the new prognostic grade group (PGG) system. Patients and Methods: Using the Shared Equal Access Regional Cancer Hospital database, we identified men after radical prostatectomy with PSA > 0.2 ng/mL and without cancer involvement of lymph nodes who underwent SRT alone. SRT failure was defined as post-SRT PSA nadir + 0.2 ng/mL or receipt of post-SRT hormone therapy. Men were stratified by pre-SRT PSA (0.2-0.49, 0.5-0.99, and ≥ 1.0 ng/mL). Multivariable Cox models were used to test the association between pre-SRT PSA and SRT failure, stratified by PGG. Results: A total of 358 men met the inclusion criteria and comprised our study cohort. Median post-SRT follow-up was 78 months. A total of 174 men (49%) had pre-SRT PSA 0.2-0.49 ng/mL, 97 (27%) PSA 0.5-0.99 ng/mL, and 87 (24%) PSA ≥ 1.0 ng/mL. On multivariable analysis among men with PGG 1-2, pre-SRT PSA 0.2-0.49 ng/mL had similar outcomes as PSA 0.5-0.99 ng/mL; those with PSA ≥ 1.0 ng/mL had higher recurrence risks (hazard ratio = 2.78, P <. 001). Among PGG 3-5, PSA 0.5-0.99 ng/mL or ≥ 1.0 ng/mL had a higher recurrence risk (hazard ratio = 2.15, P =. 021; and hazard ratio = 2.49, P =. 010, respectively) versus PSA 0.2-0.49 ng/mL. Conclusion: In men with higher-grade prostate cancer (PGG 3-5), SRT should be provided earlier (PSA < 0.5 ng/mL), while among men with lower-grade disease (PGG 1-2), SRT results in equal PSA control up to PSA 1.0 ng/mL.