Interim Prostate-Specific Antigen: Predicting for Biochemical Failure During Salvage Radiation Therapy After Prostatectomy

PURPOSE: A subset of patients treated with postprostatectomy radiation therapy for biochemical recurrence after surgery fail to respond because of microscopic disease beyond the irradiated prostate bed. This work aims to determine whether a rising interim prostate-specific antigen (PSA) during radiation therapy can predict the likelihood of subsequent biochemical recurrence. METHODS AND MATERIALS: Between 2010 and 2016, 185 patients had salvage radiation therapy to a dose of 68 Gy without androgen deprivation therapy for a rising PSA level after radical prostatectomy. Patients had their PSA recorded on the first day of radiation therapy and again after completing the 25th fraction (of 34 total fractions). Biochemical failure after radiation therapy was defined as a PSA value ≥0.2 ng/mL within 2 years after radiation therapy. Both univariate and multivariate Cox regression models were used for statistical analysis. Factors with a P value of <.2 in univariate analysis were then used in a multivariate analysis. RESULTS: The 2-year freedom from biochemical failure was 60% (95% confidence interval, 53%-67%). When assessing the interim PSA, 143 patients (77%) had a drop in interim PSA; of these patients, 71% had 2-year biochemical control. Forty-two patients (23%) had a stable or rising interim PSA, and only 24% of these patients had 2-year biochemical control. On multivariate analysis, a drop in PSA during radiation therapy (P < .0001) and a positive surgical margin (P < .0001) were significant factors for freedom from subsequent biochemical failure, and seminal vesicle invasion was associated with biochemical failure at 2 years (P = .019). All patients with a rising interim PSA, negative surgical margin, and seminal vesicle invasion ultimately had biochemical failure at 2 years. CONCLUSIONS: A PSA rise during salvage radiation therapy is prognostic of biochemical failure at 2 years. Factors such as seminal vesicle invasion and a negative surgical margin also predict for poor responders to salvage radiation therapy.