Estimation of PSA Half-Life Following Salvage Radiation Therapy

Background: We aim to estimate prostate-specific antigen (PSA) half-life after salvage radiation therapy (SRT) in patients with detectable PSA after radical prostatectomy (RP). Methods: A total of 272 patients treated with salvage radiotherapy between July 1987 and July 2010 were included in this IRB approved retrospective analysis. The median pre-salvage radiotherapy dose was 0.6 ng/mL (range, 0.2 - 21.9 ng/mL), 47 patients had at least a minimum tumor stage of T3b, 29 had a Gleason score over 7, and median dose was 66.6 Gy (range, 54.0 - 72.4 Gy). Results: The estimated PSA half-life in our cohort of patients was 3.0 months (95% CI, 2.9 - 3.2 months; range, 0.5 - 28.5 months). There was no evidence of a statistically significant association between PSA half-life and any baseline clinicopathologic characteristics. The median interval between individual PSA measurements was noted to be 4.6 months (range, 0.1 - 20.4 months). The median interval from the start of radiation therapy to the nadir PSA was 6.3 months (range, 1.3 - 79.1 months). PSA half-life remained approximately 3.0 months when accounting for infrequent and outlier PSA values. Conclusion: The PSA half-life after definitive RT has been reported to be approximately 1.6 months. Our analysis found the PSA half-life after SRT to be approximately twice that of patients treated with definitive RT. These results provide useful information to radiation oncologists when counseling patients both before and after SRT regarding expectations about PSA measurements.