LAUSR

BACKGROUND Adaptive radiotherapy has made significant advances owing to MR-LINAC, which provides superior soft-tissue contrast, fast speed and rich functional magnetic resonance imaging to guide radiotherapy. Independent dose verification plays a critical role in discovering errors, while several challenges remain in MR-LINAC. PURPOSE A Monte Carlo-based GPU-accelerated dose verification module for Unity is proposed and integrated into the commercial software ArcherQA to achieve fast and accurate quality assurance for online adaptive radiotherapy. METHODS Electron or positron motion in a magnetic field was implemented, and a material-dependent step-length limit method was used to trade off speed and accuracy. Transport was verified by dose comparison with EGSnrc in three A-B-A phantoms. Then, an accurate Monte Carlo-based Unity machine model was built in ArcherQA, including an MR-LINAC head, cryostat, coils, and treatment couch. In particular, a mixed model combining measured attenuation and homogeneous geometry was adopted for the cryostat. Several parameters in the LINAC model were tuned to commission it in the water tank. An alternating open-closed MLC plan on solid water measured with EBT-XD film was used to verify the LINAC model. Finally, the ArcherQA dose was compared with ArcCHECK measurements and GPUMCD in 30 clinical cases through the gamma test. RESULTS ArcherQA and EGSnrc were well matched in three A-B-A phantom tests, and the relative dose difference was less than 1.6% in the homogenous region. A Unity model was commissioned in the water tank, and the relative dose difference in the homogenous region was less than 2%. In the alternating open-closed MLC plan, the gamma result (3%/3 mm) between ArcherQA and Film was 96.55%, better than the gamma result between GPUMCD and Film (92.13%). In 30 clinical cases, the mean 3D gamma result (3%/2 mm) was 99.36%±1.28% between ArcherQA and ArcCHECK for the QA plans and 99.27%±1.04% between ArcherQA and GPUMCD for the clinical patient plans. The average dose calculation time was 106 s in all clinical patient plans. CONCLUSIONS A GPU-accelerated Monte Carlo-based dose verification module was developed and built for the Unity MR-LINAC. The fast speed and high accuracy were proven by comparison with EGSnrc, commission data, the ArcCHECK measurement dose and the GPUMCD dose. This module can achieve fast and accurate independent dose verification for Unity. This article is protected by copyright. All rights reserved.