Magnetic Resonance Imaging-Directed Stereotactic Body Radiation Therapy Microboost in Concert With Pelvic Nodal Irradiation for Prostate Cancer in the Higher-Risk Spectrum: Toxicity.

Purpose: Recent publications have renewed interest in prophylactic pelvic radiation therapy for higher-risk prostate cancer, as well as dose escalation for magnetic resonance imaging (MRI)-defined intraprostatic lesions. Here, we explore the use of pelvic nodal irradiation with a 3-fraction stereotactic body radiation therapy (SBRT) boost to the prostate and seminal vesicles, with a simultaneous MRI-directed focal intraprostatic lesion-ablative microboost (MIB). Methods and Materials: We evaluated an institutional registry of patients undergoing pelvic nodal radiation followed by an SBRT boost to the prostate and seminal vesicles from April 2021 to March 2023. The study was approved by the local institutional review board (study #00001269). All patients were treated with pelvic nodal irradiation followed by a 3-fraction SBRT boost. The prostate SBRT boost dose was primarily 2100 cGy in 3 fractions (an accommodated range of 1800-2100 cGy). A subgroup of 15 patients received an MIB to an additional dose of 2300 cGy in 3 fractions (range, 2100-2400 cGy). The distribution of adverse event grades for acute and late gastrointestinal (GI) and genitourinary (GU) toxicity was assessed using the National Cancer Institute Common Terminology Criteria for Adverse Events, version 5.0. Results: Fifty-eight patients underwent pelvic nodal irradiation followed by an SBRT boost to the prostate, with the distribution of risk groups as follows: patients were either in the high (36.2%, n = 21) or very high (34.5% n = 20) risk groups, whereas those with known nodal disease (19.0%, n = 11) or intermediate risk (10.3%, n = 6) comprised the rest of the study population. Most patients received androgen-deprivation therapy. The prostate SBRT boost dose was primarily 2100 cGy in 3 fractions. Fifteen patients received an MIB to an additional dose of 2300 cGy in 3 fractions. A median follow-up of 8.7 months was used to document the incidence of GU and GI toxicity. The distribution of GI and GU toxicity showed no significant difference between the MIB and non-MIB subcohorts at either the acute (<90 days) or late (>90 days) time points. Two grade 3 toxicities were observed, both in the non-MIB cohort. Grade 2+ GI and GU toxicities were not significantly different between the 2 groups, as assessed using the National Cancer Institute Common Terminology Criteria for Adverse Events, version 5.0. Conclusions: In the early follow-up period, we observed no significant difference in GI or GU toxicity between those who underwent MIB and those who did not. These results suggest that MRI-directed SBRT MIB did not increase GI toxicity and may even reduce GU toxicity compared with standard treatment. Future research should explore long-term side effects, with attention to the Expanded Prostate Cancer Index Composite (EPIC) scores and oncologic outcomes of this novel method of dose escalation.