Long-term outcomes of stereotactic body radiotherapy for low- and intermediate-risk prostate adenocarcinoma: A multi-institutional consortium study

84 Background: While a growing body of evidence supports the use of stereotactic body radiotherapy (SBRT) for the treatment of low- and intermediate-risk prostate adenocarcinoma (PCa), some trepidation exists regarding its long-term efficacy and safety. Methods: Men with low- and intermediate-risk PCa, as defined per the National Comprehensive Cancer Network guidelines, who were enrolled on various institutional phase II trials of SBRT between 2000-2012 were included in a multi-institutional consortium. Biochemical relapse (BCR) was defined as PSA > “nadir +2” or initiation of androgen deprivation therapy (ADT). Toxicity data were scored according to the CTCAE v 3.0 or Radiation Therapy Oncology Group scoring systems. Results: A total of 1644 men were eligible for analysis, with a median followup of 7.2 years. 297 patients (18.1%) had at least 9 years of followup. Fractionation schemes ranged from 33.50-40 Gy in 4-5 fractions. 892 patients had low-risk disease and 752 had intermediate-risk disease. 59 patients (3.6%) received short-term ADT. 100 patients (6.0%) experienced BCR, and 7 (0.4%) experienced distant metastases. No patients died of PCa. By Kaplan-Meier analysis, 5- and 10-year BCR-free survival rates were 98% and 94% in the low-risk group and 96% and 90% in the intermediate-risk group (p < 0.05 by log-rank test). 5- and 10-year overall survival rates were 93% and 86% in the low-risk group and 95% and 91% in the intermediate-risk group (p > 0.05 by log-rank test). Five patients (0.3%) experienced grade 3 acute genitourinary (GU) toxicities, including urinary retention, hematuria, and frequency. 30 (2%) experienced grade 3 late GU toxicity, including urinary strictures, hematuria, and retention. One late grade 4 GU toxicity (hemorrhagic urethritis) and one late grade 4 gastrointestinal toxicity (fistula-in-ano) were seen. Conclusions: To the best of our knowledge, this is the largest analysis of long-term outcomes following SBRT for PCa. The results indicate that SBRT has an efficacy and toxicity profile that compares favorably to more widespread forms of treatment, such as conventionally-fractionated external beam radiotherapy and brachytherapy.