9 Background: The purpose of this study is to report biochemical relapse-free survival (bRFS) rates for a group of localized prostate cancer patients from a pooled multi-institutional dataset with at least 5 years follow-up after stereotactic body radiotherapy (SBRT). Methods: The outcome data from 1101 patients treated with SBRT between 2003 and 2011 were pooled from 8 institutions. A subset of 135 cases had a minimum 5 years follow-up. All 135 cases had clinical stage T1 or T2A disease. The distribution by Gleason score (GS) was <6 in 80% and 7 in 20%. The median pretreatment PSA (iPSA) level was 5.1 ng/ml (range: 0.1-27.8). The distribution by risk was 77% low, 21% intermediate, and 2% high risk. The median dose was 36.25 Gy (35-40 Gy range) delivered either with 4 or 5 fractions. The prescribed dose groups were as follows: 35 Gy in 42%, 36.25 Gy in 47%, and >38 Gy in 11%. Androgen deprivation therapy was given to 21% of patients. Biochemical relapse, defined as a rise > 2 ng/ml above nadir, was determined in a total of 4 failures. Results: The median follow-up for all 135 cases was 60 months (range 60 to 72). For all patients, the bRFS rate at 5 years was 97%. The 5-year actuarial bRFS rates for GS < 6, and Gleason score 7 were 98%, and 92%, respectively (p=0.15). The 5-year actuarial bRFS rates for low versus intermediate/high-risk patients were 99% and 93%, respectively (p=0.11). The 5-year actuarial bRFS rates for patients receiving 35 Gy versus >36.25 Gy were 93% and 100%. No difference in bRFS was observed with the use of androgen deprivation (p=0.78). Multivariate analysis showed only GS to be an independent predictor of relapse (p=0.03); iPSA (p=0.10) and radiation dose (0.97) were not. Conclusions: In a relatively large cohort of localized prostate cancer patients treated with SBRT, long follow-up period (>5 years), excellent efficacy was demonstrated with 97% of patients being free from relapse. For low and intermediate risk cases, these results compare favorably with other modalities with similar follow-up periods. Although a trend for worse outcome was seen with total radiation doses of 35 Gy, this was not confirmed on multivariate analysis.
