Stereotactic Body Radiotherapy Boost Toxicity For High And Intermediate-Risk Prostate Cancer: Report Of A Multi-Institutional Study

365 Background: External beam radiation therapy (EBRT) with androgen suppression (AS) and low dose rate (LDR)brachytherapy boost has been shown to improve biochemical progression free survival in unfavorable intermediate risk and high-risk prostate cancer (PC) compared to EBRT and AS. Excellent rates of local control in locally advanced prostate cancer with EBRT with high dose rate brachytherapy (HDR) boost. Prostate Stereotactic Radiosurgery (SBRT) may be an alternative to brachytherapy in patients with unfavorable intermediate and high-risk PC. Here we report the toxicity of pelvic lymph node/prostate EBRT and SBRT as the radiation boost in a large retrospective cohort. Methods: 473 patients with intermediate or high-risk PC, from the Radiosurgery Society Registry, Beth Israel Deaconess Medical Center, Georgetown University, and 5 Australian centers, were included. Patients received treatment from 3/2004- 9/2018 were the basis of this IRB approved retrospective study. The prostate and pelvis nodes were treated with between 36-50.4 Gy in 1.8/2.0 Gy fractions of radiation therapy EBRT. Patients received a SBRT boost to the prostate. Boost dose was 19-19.5 Gy (range 19-36.25 Gy). 274 and 199 patients presented with unfavorable or high-risk disease. The median follow-up was 33 months (IQR:18-63). Results: 33 deaths of 473 patients occurred in this cohort, 8 of which were caused by PC. There were 13.9% (n=66) acute Grade 1 or 2 GI toxicities (11.8% grade 1, 2.1% grade 2). There were 27.7% (n=131) acute Grade 1 or 2 GU toxicities (19.2% grade 1, 8.5% grade 2). No severe acute GU or GI toxicities were reported. There were 32 (6.8%) Grade 1 and 3 (0.6%) Grade 2 late GI toxicities. There were 9 (1.9%) Grade 3 and 1 (0.2%) Grade 4 late GI toxicities. There were 60 (12.7%) Grade 1 and 23 (4.9%) Grade 2 late GU toxicities. 15 (3.2%) Grade 3, but no Grade 4 late GU toxicity were reported. Conclusions: In this large multi-institutional retrospective cohort SBRT boost to pelvic/prostate EBRT had acceptable acute and late toxicities. The high dose per fraction of SBRT is similar to the dose delivered with HDR. This data raises the hypothesis that SBRT boost should be evaluated in additional clinical trials.