Multi-institutional analysis of high-risk prostate cancer patients treated with stereotactic body radiotherapy.

Abstract

51 Background: Stereotactic Body Radiotherapy (SBRT) delivers ablative doses of radiation (RT) over a course of five treatments and has been increasingly used as a definitive RT option for low- and intermediate-risk prostate cancer (PCa). Ongoing prospective trials are evaluating the efficacy of SBRT for high-risk PCa, but clinical outcomes reports are limited. Methods: Patients treated for high-risk PCa between 2006-2017 at any of five institutions were included. SBRT doses ranged from 35-40 Gy in 5 fractions per institutional standards, with one institution using an integrated boost approach. The Phoenix definition was used to define biochemical failure (BCR). Physician-reported genitourinary (GU) and gastrointestinal (GI) toxicity outcomes were scored using the Radiation Therapy Oncology Group or Common Terminology Criteria for Adverse Events systems. Results: In total, 182 patients were included in this study with a median follow-up time of 38.4 months (mos). The median age was 72. Most patients (72%) had Gleason 8-10 disease. Sixty-eight percent of patients received androgen deprivation therapy (ADT) for a median of 9 mos (interquartile range 6-9 mos). The rate of distant metastases was 3.8%. There were no acute Grade 3 (G3) or higher GU or GI toxicities. Three patients (1.6%) experienced a late G3 GU toxicity and one patient (0.5%) experienced a late G3 GI toxicity. The incidence of BCR was significantly higher in patients who did not receive ADT (30% vs. 15%, p = 0.02 by Chi-square). Conclusions: In this multi-institutional study, SBRT demonstrated an acceptable safety profile for the treatment of high-risk PCa. Longer term follow-up is necessary to evaluate the oncologic efficacy of this approach, but given the potentially higher incidence of BCR without ADT, ADT likely has an important oncologic role even with SBRT regimens.