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

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.

This study establishes and demonstrates a framework of commissioning tests to facilitate clinical implementation of VMAT on the MR-linac.

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.

All static beam profiles met clinical tolerances (symmetry <3%, flatness <3%). Profiles remained stable during dynamic delivery for all gantry speeds (all gamma passing rates> 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.

The presented tests provide a foundation for VMAT commissioning for the MR-linac that verifies the stability of the mechanical, radiation, and imaging components.