In Silico Single-Fraction Stereotactic Ablative Radiation Therapy for the Treatment of Thoracic and Abdominal Oligometastatic Disease With Online Adaptive Magnetic Resonance Guidance

PURPOSE: Although SABR can improve oncologic outcomes for patients with oligometastatic disease, treatment of metastases near critical organs remains challenging. The purpose of this study is to determine the dosimetric feasibility of delivering magnetic resonance imaging (MRI)-guided adaptive SABR in a single fraction for abdominal and thoracic metastases. METHODS AND MATERIALS: Previously delivered MRI-guided radiation therapy plans for 20 patients with oligometastatic disease in the thorax or abdomen, with 70% (14/20) of the lesions within 8 mm from dose-limiting organs at risk (OARs), were used to simulate the delivery of 24 Gy in a single fraction. Planning objectives included planning target volume (PTV) V(95%) >90%, optimized PTV (PTVopt) V(95%) >90%, and PTVopt D(99%) >20 Gy with no OAR dose violations, where PTVopt removed overlap with nearby planning organ at risk volume (PRV). Single-fraction plans were simulated on the first 5 daily setup breath-hold MRI scans, and the plans were reoptimized to consider variations in setup position and anatomy. RESULTS: The mean PTV V(95%) for single-fraction SABR plans was lower compared with multifraction plans (mean 85.4% vs 92.6%, P = .02), but mean PTVopt V(95%) was not different (95.3% vs 98.2%, P = .62). After reoptimization of the single-fraction plan to the treatment day MRI, there was an increase in mean PTV V(95%) (85.0% vs 88.1%, P = .05), increase in mean PTVopt V(95%) (92.7% vs 96.3%, P = .02), increase in mean PTVopt D(99%) (19.7 Gy vs 23.8 Gy, P < .01), increase in mean frequency of meeting PTV D(99%) >20 Gy (52% vs 87%, P < .01), and increase in mean gross tumor volume minimum dose (17.5 Gy vs 19.3 Gy, P < .01). Reoptimization decreased mean frequency of OAR dose constraint violation (48% vs 0%, P < .01). CONCLUSIONS: Single-fraction MRI-guided SABR is a dosimetrically feasible treatment for oligometastases that allows for on-table adaptation to avoid OAR dose constraint violations, but this method requires clinical validation.