Purpose: In MR-guided radiotherapy, there exists an unmet need for robust, 3D scanning equipment that can be used for data acquisition during acceptance/commissioning and periodic QA testing. A vendor-provided MR-conditional prototype 3D scanning system was recently developed and tested on a clinical MR-guided Linac.
Methods: Tests were performed using a 0.35T MR-Linac delivering a 6MV flattening-filter-free beam with a bore diameter of 70cm. The certified MR-conditional scanning system comprises web-based software, a water reservoir, an electronics control box, a hand pendant, and a 3D rectangular water tank. Scanning was performed over three sessions using 85 and 80cm SSDs, 5 and 10cm depths, and field sizes (FS) ranging from 0.4x0.2cmÂ² to 27.2x24.1cmÂ². MR-compatible A26 (field) and A1SL (reference) ionization chambers were used for FS >2.5x2.5cmÂ² and an Edge diode detector for smaller FS. For each session, inline and crossline profiles as well as percent depth dose (PDD) measurements were acquired. Profiles and PDDs were compared across sessions and to Monte Carlo simulations and commissioning measurements acquired using either a 1D tank with manual changes in detector position applied between measurements or 2D ion chamber array.
Results: For PDDs at 10x10cmÂ² FS, dmax was constant and PDD10x was within 1% (mean: 1.4cm and 61.6%, respectively) across all sessions. At 80cm SSD, dmax was within 0.5mm and PDD10x within 1% of baseline commissioning measurements. Both crossline and inline flatness and symmetry at 10x10cmÂ² and 27.2x24.1cmÂ² FS were within 1% of baseline measurements acquired during commissioning and field widths for all FS were within 1mm of nominal values.
Conclusion: Performance of a novel, MR-compatible 3D scanning water tank was assessed in a clinical MR-Linac system. Profile and PDD measurements were acquired using ion chambers and diodes. Data was consistent between scanning sessions and with previously acquired commissioning data.
Funding Support, Disclosures, and Conflict of Interest: This work was supported by LAP GmbH Laser Application. Henry Ford holds research agreements with Viewray, Inc.