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

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 350mm-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.

Influence of beam quality on reference dosimetry correction factors in magnetic resonance guided radiation therapy

“…For the 7 MV FFF beam quality, the simulated beam quality specifier is in excellent agreement with experimental results that can be found in the literature [12] , [23] . De Prez et al [12] found and Snyder et al [23] published a value of . In contrast, the results of this work do not agree with the Monte Carlo simulated values of other authors [9] , [10] .…”

Imaging performance of a high-field in-line magnetic resonance imaging linear accelerator with a patient rotation system for fixed-gantry radiotherapy

“…In this paper we have described our initial findings with respect to image quality using a prototype patient rotation system for a high-field in-line MRI-Linac. With the PRS in-situ, geometric distortion was within 1.6 ppm, which could likely be accounted for with active shimming, and is consistent with commercial MRI-Linac systems for a DSV of 20 cm [13,14]. A previous study by Shan et al quantified the B0-field inhomogeneity of the MRI-Linac used in this study, using the same B0 mapping approach [15].…”

Analysis of patient‐specific quality assurance for Elekta Unity adaptive plans using statistical process control methodology

“…Due to the design of the Unity MR-linac, the ArcCheck phantom is first placed on the QA platform and translated into the bore. A CT scan of the ArcCheck was imported into the Monaco treatment planning system(TPS) and the relative electron densities were determined as described by Snyder et al14,15 The alignment of the ArcCheck phantom utilizes a custom QA platform since the Unity MR-linac only has a sagittal laser, which is insufficient to properly align the phantom by itself. As a result, the alignment reproducibility is dependent on the consistency of how the ArcCheck phantom and platform are setup on the treatment couch.…”