Multi-institutional Analysis of MR-Guided Single-Fraction Stereotactic Ablative Body Radiation Therapy: Feasibility, Safety, and Efficacy.

Abstract

Stereotactic ablative body radiation therapy (SABR) is commonly delivered in multiple fractions, whereas single-fraction (SF) SABR remains uncommon. Magnetic resonance (MR)-guided linacs provide superior soft-tissue contrast compared to cone-beam computed tomography on conventional linacs, online adaptive replanning, continuous intrafraction visualization, and automatically triggered beam delivery, potentially enabling safe and effective SF-SABR. We retrospectively analyzed patients treated with MR-guided SF-SABR on a 0.35 Tesla MR-linacs (ViewRay Inc, MRIdian) at 2 centers in Germany and the United States. Median SF-SABR doses were 28 Gy (range, 28-40 Gy) for liver lesions, 30 Gy (range, 28-34 Gy) for lung lesions, 25 Gy for adrenal gland lesions and lymph nodes, and 26 Gy (range, 25-26 Gy) for renal lesions. Endpoints included local control (LC), progression-free survival, overall survival (OS), and toxicity graded according to Common Terminology Criteria for Adverse Events version 5.0. Between April 2020 and December 2024, 161 patients with 175 lesions underwent MR-guided SF-SABR. Lesion distribution was liver (48.0%), lung (42.9%), adrenal gland (4.3%), lymph nodes (3.4%), and kidney (1.9%). Median gross tumor volume and planning target volume were 2.4 cc (range, 0.2-83.3 cc) and 9.3 cc (range, 1.4-112.1 cc), respectively. At a median follow-up of 19.3 months, LC rates at 1 and 2 years were 99.1% and 93.6%, respectively. Progression-free survival rates at 1 and 2 years were 59.3% and 51.4%, while OS rates were 81.8% and 73.4%, respectively. Treatment was well tolerated, with only one acute grade 3 toxicity reported and no acute grade 4-5 or late grade 3-5 toxicities. Colorectal primary tumors were associated with poorer LC (hazard ratio [HR] = 0.226; P = .023). Larger gross tumor volume independently correlated with reduced LC (HR = 1.15; P = .048) and OS (HR = 1.05; P = .001). MR-guided SF-SABR is feasible, safe, and effective across various tumor locations, providing favorable LC with minimal toxicity, warranting further prospective evaluation.