Stereotactic Body Radiation Therapy (SBRT) for Intermediate and High-Risk Prostate Cancer: Early Adoption in a Community Hospital Setting

Abstract

The primary objective of the current study is to report long-term survival (OS), biochemical relapse free survival (BRFS) and toxicity following SBRT for men with intermediate and high risk prostate cancer in a community hospital setting. Ninety-eight consecutive men treated with SBRT between 2007 and 2015 were retrospectively stratified by NCCN risk groups, including favorable intermediate (n=43), unfavorable intermediate (n=44) and high risk (n=11). Additional factors for analysis include Gleason Score (GS), clinical stage, initial PSA, % of cores positive (<50% vs >50%), PTV dose (35Gy vs 36.25Gy vs 37.5Gy), use of ADT, race, and age. Biochemical failure was assessed using the Phoenix nadir +2 definition. Toxicity was assessed using the Radiation Therapy Oncology Group (RTOG) criteria with modification for bowel and bladder function. OS was calculated from end of SBRT to date of death or last follow-up (FU). BRFS was calculated from end of SBRT to date of biochemical failure or most recent PSA. OS and BRFS were estimated using Kaplan-Meier methodology with comparisons accomplished using log-rank statistics. Median FU is 58 months. Median initial PSA is 6.2 (range 1.3-62). Median PSA nadir is 0.14 with interquartile range of 0.10 to 0.40. Nine-year actuarial OS is 78.3%. No significant differences in OS are observed between risk groups or other prognostic/treatment factors. Eight-year actuarial BRFS is 86%. Significant differences in BRFS are observed between GS (p=0.0003), with a 7-year BRFS of 100% for GS 3+3/3+4 versus 60.5% for GS 4+3/4+4. BRFS is significantly higher for those with favorable intermediate risk with 7-year BRFS of 100% compared with 78.2% for unfavorable intermediate and high risk prostate cancer (p=0.0425). Significant differences are also noted in BRFS with 100% for high dose (37.5Gy) versus 84.9% for intermediate dose (36.25Gy) at 7 years (p=0.009). Too few patients are treated with 35Gy (n=2) for 7-year estimates. Multivariable analysis reveals GS as the sole predictor (p=0.0144) of BRFS, with 93% decreased risk for failure for GS 3+3/3+4 compared with GS 4+3/4+4. Cumulative incidence of late RTOG grade 3 urinary toxicity is 5%. Cumulative incidence of late RTOG grade 3 bowel toxicity is 1%. Eight percent of men with no erectile dysfunction at presentation developed erections insufficient for intercourse most of the time despite use of oral medications. SBRT may be an acceptable alternative for men with intermediate and high risk prostate cancer. Gleason score is the single most important independent factor for BRFS. Outcome for favorable intermediate risk prostate cancer is excellent however unfavorable intermediate prostate cancer behaves like high risk disease. Further study regarding the benefit of higher doses of radiation, in particular for high risk patients, should be explored.