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Technical note: A simple method for patient-specific quality assurance for lateral targets on a 1.5 T MR-Linac
Journal article   Open access   Peer reviewed

Technical note: A simple method for patient-specific quality assurance for lateral targets on a 1.5 T MR-Linac

William S Ferris, Blake R Smith, Daniel E Hyer and Joel J St-Aubin
Journal of applied clinical medical physics, Vol.25(4), e14323
04/2024
DOI: 10.1002/acm2.14323
PMCID: PMC11005970
PMID: 38426612
Appears in  UI Libraries Support Open Access
url
https://doi.org/10.1002/acm2.14323View
Published (Version of record) Open Access

Abstract

The Elekta Unity magnetic resonance (MR) linac is limited to longitudinal couch motion and a sagittal-only laser, which restricts the ability to perform patient-specific quality assurance (PSQA) intensity-modulated radiotherapy (IMRT) measurements for very lateral targets. This work introduces a simple method to perform PSQA using the Sun Nuclear ArcCheck-MR phantom at left and right lateral positions without additional equipment or in-house construction. The proposed setup places the center of the phantom 1.3 cm vertical and 12.9 cm lateral to isocenter in either the left or right direction. Computed tomography (CT) scans are used to simulate the setup and create a QA plan template in the Monaco treatment planning system (TPS). The workflow is demonstrated for four patients, with an average axial distance from the center of the bore to the planning target volume (PTV) of 12.4 cm. Gamma pass rates were above 94% for all plans using global 3%/2 mm gamma criterion with a 10% threshold. Setup uncertainties are slightly larger for the proposed lateral setup compared to the centered setup on the Elekta platform (∼1 mm compared to ∼0.5 mm), but acceptable pass rates are achievable without optimizing shifts in the gamma analysis software. In general, adding the left and right lateral positions increases the axial area in the bore encompassed by the cylindrical measurement array by 147%, substantially increasing the flexibility of measurements for offset targets. Based on this work, we propose using the lateral QA setup if the closest distance to the PTV edge from isocenter is larger than the array radius (10.5 cm) or the percent of the PTV encompassed by the diode array would be increased with the lateral setup compared to the centered setup.
 unity ArcCheck PSQA QA platform 1.5T MR Linac UIOWA OA Agreement

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