Stereotactic Magnetic Resonance Guided Adaptive Radiation Therapy (SMART) for Abdominopelvic Oligometastases: A Combined Analysis of Prospective and Retrospective Cohorts

Purpose/Objective(s)Stereotactic body radiation therapy can be an effective treatment for oligometastases. However, safe delivery of ablative radiation therapy is commonly limited by the proximity of mobile organs at risk (OARs), as interfraction and intrafraction motion can result in severe complications. The goal of this study is to determine the feasibility, safety, and disease control outcomes of stereotactic magnetic resonance guided adaptive radiation therapy (SMART) in patients with abdominopelvic oligometastases.Materials/MethodsWe identified 101 patients with abdominopelvic oligometastases, including 20 patients enrolled on phase 1 protocols, who were consecutively treated with SMART on a 0.35 T MR Linac at a single institution from October 2019 to September 2021, excluding patients with metastases of the liver, kidneys, or adrenal glands. Local control and overall survival were analyzed using the Kaplan-Meier method. Adverse effects were assessed using the Common Terminology Criteria for Adverse Events (CTCAE) v5.0.ResultsOverall, 114 sites of disease were treated. The most common histology was prostate adenocarcinoma (53.5%, n=60), and 57.0% (n=65) of sites were centered in the pelvis. 79.8% of sites (n=91) were treated to 8 Gy × 5, and 43.0% (n=49) were treated with breath hold. Online adaptation was used in 86.6% of delivered fractions to either improve the sparing of OARs (57.2%) or target coverage (29.4%). In addition, there were two cases where the on-board MRI allowed for the detection of bowel which had displaced the target to such a degree as to interfere with safe treatment delivery. These two fractions were postponed to either later the same day or a subsequent day. 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 higher for prostate adenocarcinoma vs other histologies (100% vs 84%, log-rank P=0.009). The 12-month overall survival was 96% and higher for prostate adenocarcinoma vs other histologies (100% vs 91%, log-rank P=0.046). Three patients developed grade 3 toxicities (colonic hemorrhage at 3.4 months and urinary tract obstructions secondary to ureteral stenoses at 10.1 and 18.4 months).ConclusionSMART is a feasible and safe method for delivering ablative radiation therapy to abdominopelvic oligometastases. Early data indicate a high rate of local control and low risk of significant toxicity. 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.