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Commissioning of a 1.5T Elekta Unity MR-linac: A single institution experience
Journal article   Open access   Peer reviewed

Commissioning of a 1.5T Elekta Unity MR-linac: A single institution experience

Jeffrey E. Snyder, Joel St-Aubin, Sridhar Yaddanapudi, Amanda Boczkowski, David A. P. Dunkerley, Stephen A. Graves and Daniel E. Hyer
Journal of applied clinical medical physics, Vol.21(7), pp.160-172
07/01/2020
DOI: 10.1002/acm2.12902
PMCID: PMC7386194
PMID: 32432405
url
https://doi.org/10.1002/acm2.12902View
Published (Version of record) Open Access

Abstract

MR image-guided radiotherapy has the potential to improve patient care, but integration of an MRI scanner with a linear accelerator adds complexity to the commissioning process. This work describes a single institution experience of commissioning an Elekta Unity MR-linac, including mechanical testing, MRI scanner commissioning, and dosimetric validation. Mechanical testing included multileaf collimator (MLC) positional accuracy, measurement of radiation isocenter diameter, and MR-to-MV coincidence. Key MRI tests included magnetic field homogeneity, geometric accuracy, image quality, and the accuracy of navigator-triggered imaging for motion management. Dosimetric validation consisted of comparison between measured and calculated PDDs and profiles, IMRT measurements, and end-to-end testing. Multileaf collimator positional accuracy was within 1.0 mm, the measured radiation isocenter walkout was 0.20 mm, and the coincidence between MR and MV isocenter was 1.06 mm, which is accounted for in the treatment planning system (TPS). For a 350-mm-diameter spherical volume, the peak-to-peak deviation of the magnetic field homogeneity was 4.44 ppm and the geometric distortion was 0.8 mm. All image quality metrics were within ACR recommendations. Navigator-triggered images showed a maximum deviation of 0.42, 0.75, and 3.0 mm in the target centroid location compared to the stationary target for a 20 mm motion at 10, 15, and 20 breaths per minute, respectively. TPS-calculated PDDs and profiles showed excellent agreement with measurement. The gamma passing rate for IMRT plans was 98.4 +/- 1.1% (3%/ 2 mm) and end-to-end testing of adapted plans showed agreement within 0.4% between ion-chamber measurement and TPS calculation. All credentialing criteria were satisfied in an independent end-to-end test using an IROC MRgRT phantom.
 Life Sciences & Biomedicine Radiology, Nuclear Medicine & Medical Imaging Science & Technology

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