C Glide-Hurst¹*, P Yadav² , A Gutierrez³ , G Gungor⁴ , D Hoffmans⁵ , K Mittauer² , A Sethi⁶ , T Romaguera³ , R Pennell⁷ , M Palacios⁵ , E Omari⁶ , S Meeks⁸ , H Mailleux⁹ , O Green¹⁰ , P Fau⁹ , A Doemer¹ , J.K. DeWyngaert⁷ , K Boye¹¹ , A Shah⁸ , S Klueter¹² , R Lotey¹³ , M Bellon¹³ , (1) Henry Ford Health System, Detroit, MI, (2) School of Medicine and Public Health UW-Madison, Madison, WI, (3) Miami Cancer Institute, Miami, FL, (4) INSTITUTE OF HEALTH SCIENCE OF ACIBADEM UNIVERSITY, Istanbul, (5) Amsterdam University Medical Center, Amsterdam, Netherlands, (6) Loyola Univ Medical Center, Maywood, IL, (7)New York Presbyterian, New York, NY, (8) UF Health Cancer Center at Orlando Health, Orlando, FL, (9) Institut Paoli-Calmettes, Marseille, France,(10) Washington University School of Medicine, St. Louis, MO, (11) Rigshospitalet Copenhagen, Denmark,(12) Heidelberg University Hospital, Heidelberg, Germany, (13) ViewRay Incorporated, Mountain View, CA

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  C Glide-Hurst

(Sunday, 7/14/2019) 5:00 PM - 6:00 PM

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 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 ≤0.8mm 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 <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 ± 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.

Keywords

Linear Accelerator, MR, Quality Assurance

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined Quality Assurance

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