Development and validation of commissioning tests for volumetric modulated arc therapy on the 1.5T MR-linac

Background: The MR-Linac enables adaptive radiotherapy using MRI guidance. Volumetric modulated arc therapy (VMAT) has been shown to be feasible on the MR-Linac and offers many advantages, but commissioning tests are needed. Purpose: This study establishes and demonstrates a framework of commissioning tests to facilitate clinical implementation of VMAT on the MR-linac. Methods: A framework of tests was developed to verify stability and accuracy of the radiation, mechanical and imaging systems during dynamic delivery for MR-linac platforms. Beam stability is assessed under static and dynamic gantry conditions using the portal imager and detector array. The beam-limiting device (BLD) is evaluated through sliding-window measurements and a modified strip test to determine its suitability for dynamic delivery. MRI stability during gantry rotation is examined using B0 and B1 mapping, geometric fidelity phantoms, and isocenter alignment tests. All tests were performed on two MR-linacs to demonstrate feasibility. Results: All static beam profiles met clinical tolerances (symmetry 95%). The BLD is sufficiently well modeled in the planning system, and strip tests showed accurate leaf positioning and dynamic stability. MRI stability measurements demonstrated minimal B0, and B1 variations, and geometric fidelity was within tolerances. No degradation in image quality was observed during gantry rotation. Conclusions: The presented tests provide a foundation for VMAT commissioning for the MR-linac that verifies the stability of the mechanical, radiation, and imaging components.