Benchmarking of Low-Field MR-Linacs in a Multi-Institutional International Consortium

Purpose: Recently, MR-linacs have been integrated into clinical practice, introducing new needs for QA and baseline machine characterization. This work summarizes a multi-institutional evaluation 12 low-field MR-linacs with the overarching goal of benchmarking machine performance. Methods: Acceptance and commissioning data were analyzed for 12 0.35 T ViewRay MRIdian linacs equipped with double-focused MLCs (4 mm aperture resolution). MRI-radiation isocenter accuracy was assessed. Couch transmission was measured at various beam angle incidences. Dosimetric evaluation included 6XFFF photon beam spot size, profiles, PDD curves, chamber-corrected Monte-Carlo derived relative photon OFs (0.83-25.6 cm2 field sizes), temporal output factor stability, and MLC transmission/leakage. End-to-end testing and IMRT performance were evaluated. MRI benchmarking included spatial integrity, magnetic field homogeneity (MFH) using spectral peak analysis (5-12 gantry angles), and image quality evaluation via ACR/NEMA standards. Clinical integration including QA timelines, staffing, and equipment were summarized. Results: MRI/laser/radiation isocenter coincidence was ≤0.8 mm for all MR-linacs. Couch transmission ranged from 13% to 17% (180° and 140°, respectively) requiring inclusion in treatment planning. Excellent agreement in PDD(10)x was observed (64.1 ± 0.4%) with spot sizes of 0.15 ± 0.03 mm. The largest discrepancy in corrected OFs was 0.72 ± 0.03 (0.83 cm2 field size) while all other OFs were in close agreement. Average output values within 2-18 months of initial calibration were \u3c1% of nominal; four institutions adjusted output at ∼90 days. On average, MLC transmission and leakage were \u3c0.3% and all IMRT plans were within 99% agreement of expected (3%/3 mm). MRI ACR and vendor-specified limits were met for all image quality metrics. Gantryangle dependence of MFH was observed (2.93 ± 1.82 ppm) with 3/12 institutions exceeding 5 ppm at a subset of angles, warranting a dynamic gantry angle-dependent shim. Conclusion: Overall, excellent agreement in multiinstitutional commissioning data was observed, providing important comparison data to others embarking on MR-linac commissioning