Purpose: Stereotactic body radiation therapy (SBRT) is an advanced technique that enables precise delivery of radiation directly to a tumor, typically in ≤5 fractions. Single-fraction SBRT for visceral tumors is uncommon, likely related to concerns about risks of geographic tumor miss because of suboptimal cone beam computed tomography scan quality on conventional linear accelerators (linacs). Magnetic resonance (MR) guided linacs are a novel technology offering superior imaging that might facilitate the safe delivery of single-fraction SBRT. Methods and Materials: We conducted a multicenter phase 2 trial of single-fraction SBRT delivered on a 0.35 Tesla MR-linac for primary or metastatic lesions of the lung (30-34 Gy; biologically effective dose [BED10] = 120-149.6 Gy10), liver (35-40 Gy; BED10 = 157.5-200 Gy10), pancreas (25 Gy; BED10 = 87.5 Gy10), adrenal gland (25 Gy10), kidney (25 Gy10), and abdominal/pelvic lymph nodes (25 Gy10). Primary objectives included feasibility and safety. The trial is registered with ClinicalTrials.gov, NCT04939246. Results: The study accrued 30 patients with 32 lesions at 2 centers in the United States between June 2021 and June 2023. All patients had 1 lesion except for 2 with 2 lesions each. Target locations included lung (34.4%), adrenal gland (28.1%), lymph node (18.8%), liver (15.6%), and pancreas (3.1%). The primary objectives were met; total in-room time was <90 minutes for 87.1% of delivered plans and 1 acute grade 3 adverse event was possibly related to single-fraction SBRT. No late grade 3-to-5 adverse events were observed. One-year local control and overall survival were 96.2% (95% CI, 88.8%-100%) and 86.3% (95% CI, 73.8%-98.8%), respectively. Conclusions: This is the first prospective study to demonstrate that MR guided single-fraction SBRT is feasible, safe, and effective for not only tumors in the peripheral lung, but also the abdomen and pelvis. Future studies should clarify patient selection for single- versus multifraction SBRT.
