An efficient method to evaluate the mechanical accuracy of MR-Linac configured with an MV panel.

Background: The mechanical accuracy of magnetic resonance linear accelerators (MR-Linac) is crucial in terms of the accuracy of magnetic resonance-guided radiotherapy. Current clinical quality assurance procedures, which involve individual measurements of the accelerator's mechanical components, have high time costs.

Purpose: This study developed an efficient method to evaluate the mechanical accuracy of the MR-Linac by measuring multiple mechanical components simultaneously using a single phantom with only one setup.

Methods: The measurements were performed using an MR-to-MV phantom with an Elekta Unity MR-Linac. The phantom contains regularly arranged ceramic ball bearings (BB) that are visible in megavoltage (MV) images. MV projection images of the phantom were acquired at various gantry angles, and a software program was developed to detect the radiation field edges and the positions of the BBs, thereby enabling effective and efficient measurement of the radiation isocenter size, field size accuracy, couch position accuracy, and gantry angle accuracy. The accuracy, reproducibility and robustness of the proposed method were evaluated through tests with different gantry angles and phantom offsets.

Results: The entire measurement procedure was completed in 6.3 ± 0.2 min, and the obtained results were consistent with those of conventional methods. The proposed method can detect angular uncertainties as small as 0.1°. The measurement results exhibited excellent inter-operator reproducibility, with the intraclass correlation coefficient >0.9 and standard deviations within 0.1 mm and 0.1°. In the robustness test, introducing a 2 mm phantom setup error resulted in an average deviation of 0.02 mm in the measured radiation isocenter size and a maximum deviation of approximately 0.1° in the gantry angle.

Conclusion: The proposed is a simple, robust, and accurate tool to measure the mechanical accuracy of an MR-Linac. By enabling simultaneous measurement of multiple mechanical parameters using a single phantom, the proposed method reduces the time costs of quality assurance procedures considerably.