Predicting Biochemical Disease-Free Survival after Prostate Stereotactic Body Radiotherapy: Risk-Stratification and Patterns of Failure

BACKGROUND AND PURPOSE: To determine appropriate risk-stratification factors for prostate cancer patients undergoing stereotactic body radiotherapy (SBRT). MATERIALS AND METHODS: Between 2006 and 2010, 515 patients with organ-confined prostate cancer were treated with a regimen of five-fraction SBRT to dose of 35–36.25 Gy. By NCCN criteria, 324 patients were low risk, 153 were intermediate risk, and 38 were high risk. Patients were defined as unfavorable intermediate risk if Gleason 4 + 3 = 7 or >1 intermediate-risk factors (cT2b, c, PSA 10–20, Gleason 3 + 4 = 7). Cox regression analysis was used to determine risk factors significantly associated biochemical failure, and patterns of failure analyzed. RESULTS: With median follow-up of 84 months, the 8-year disease-free survival was 93.6, 84.3, and 65.0% for low, intermediate, and high-risk group patients, respectively. Based on the above definition, 106 favorable intermediate-risk patients had excellent outcomes, with no significant difference compared to low-risk patients (7-year DFS 95.2 vs. 93.2%, respectively). The 47 unfavorable intermediate-risk patients had worse outcomes, similar to high-risk patients (7-year DFS 68.2 vs. 65.0%, respectively). Gleason score was the only significant factor associated with biochemical failure on multivariate analysis (p = 0.0003). CONCLUSION: Patients with favorable intermediate-risk disease have excellent outcomes, comparable to low-risk patients. Patients with unfavorable intermediate-risk disease have significantly worse outcomes after SBRT, and should be considered for clinical trials or treatment intensification.