Stereotactic Magnetic Resonance-Guided Adaptive Radiation Therapy (SMART) for Abdominopelvic Oligometastases

Purpose Stereotactic body radiation therapy can be an effective treatment for oligometastases. However, safe delivery of ablative radiation is frequently limited by the proximity of mobile organs sensitive to high radiation doses. The goal of this study was to determine the feasibility, safety, and disease control outcomes of stereotactic magnetic resonance–guided adaptive radiation therapy (SMART) in patients with abdominopelvic oligometastases. Methods and Materials We identified 101 patients with abdominopelvic oligometastases, including 20 patients enrolled on phase 1 protocols, who were consecutively treated with SMART on a 0.35T magnetic resonance linear accelerator (MR linac) at a single institution from October 2019 to September 2021. Local control and overall survival were analyzed using the Kaplan-Meier method. Results Overall, 114 tumors were treated. The most common histology was prostate adenocarcinoma (60 tumors [53.5%]), and 65 sites (57.0%) were centered in the pelvis. Ninety-one sites (79.8%) were treated to 8 Gy × 5, and 49 (43.0%) were treated with breath-hold respiratory gating. Online adaptation resulted in a clinically significant improvement in coverage or organ sparing in 86.6% of delivered fractions. The median time required for adaptation was 24 minutes, and the median time in the treatment room was 58 minutes. With median follow-up of 11.4 months, the 12-month local control was 93% and was higher for prostate adenocarcinoma versus other histologies (100% vs 84%; P = .009). The 12-month overall survival was 96% and was higher for prostate adenocarcinoma versus other histologies (100% vs 91%; P = .046). Three patients (3.0%) developed grade 3 toxic effects (colonic hemorrhage at 3.4 months and urinary tract obstructions at 10.1 and 18.4 months, respectively). Conclusions In this study, SMART was feasible, safe, and effective for delivering ablative radiation therapy to abdominopelvic metastases. Adaptive planning was necessary in the large majority of cases. The advantages of SMART warrant its further investigation as a standard option for the treatment of abdominopelvic oligometastases.