Room: Stars at Night Ballroom 4
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 0.35T ViewRay MRIdian linacs equipped with double-focused MLCs (4mm 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 cmÂ² 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 â‰¤0.8mm for all MR-linacs. Couch transmission ranged from 13 to 17% (180Â° and 140Â°, respectively) requiring inclusion in treatment planning. Excellent agreement in PDD(10)x was observed (64.1 +/- 0.4%) with spot sizes of 0.15 Â± 0.03 mm. The largest discrepancy in corrected OFs was 0.72Â±0.03 (0.83 cmÂ² field size) while all other OFs were in close agreement. Average output values within 2-18 months of initial calibration were <1% of nominal; four institutions adjusted output at ~90 days. On average, MLC transmission and leakage were <0.3% and all IMRT plans were within 99% agreement of expected (3%/3mm). MRI ACR and vendor-specified limits were met for all image quality metrics. Gantry-angle dependence of MFH was observed (2.93 Â± 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 multi-institutional commissioning data was observed, providing important comparison data to others embarking on MR-linac commissioning.
Funding Support, Disclosures, and Conflict of Interest: Authors from submitting institutions have funding, speaking honoraria, consulting fees, and travel expenses by ViewRay, Inc, not related to the current initiative. C. Glide-Hurst discloses research agreements with Philips Healthcare, ViewRay, Inc., and Modus Medical. Research partially supported by the NCI/NIH, Award R01CA204189.