Room: Track 2
Purpose: To measure output, symmetry, field centering and multileaf-collimator (MLC) position with high sensitivity using scintillation imaging for MR-Linac Quality Assurance.
Methods: This study was performed using a 6 MV FFF beam from a 0.35 T MR-linac. An intensified CMOS camera was mounted on the wall at a distance of 5.5 m to isocenter and time-gated to the linac pulses. A scintillation screen was placed at isocenter on a 2 cm slab of solid water in the x- and y-directions, with the z-position lowered to provide the best camera view. Varying monitor units were delivered to test output response, and various field sizes were used measure crossplane and inplane profiles. Shifts from -3 to +3 mm in the x-direction were programmed using the MLC to characterize sensitivity to small spatial deviations. Lastly, an MLC picket fence test was analyzed to verify the MLC positions.
Results: The method, featured with MR-compatibility and set-up simplicity, was found to be sensitive to deviations in output of 0.5%, and the signal was linear with the number of MUs delivered (R² = 0.99). Agreement between programmed and measure field sizes ranged from 0.1 mm to 1.3 mm. Measured shifts from the optical images matched the programmed field shifts in the x-direction with agreement ranging from 0.2 mm to 1.0 mm. MLC positions from the picket fence test matched with < 2 mm discrepancy.
Conclusion: This method can provide valuable output and symmetry information for consistency measurements on an MR-linac. The system shows high sensitivity to shifts in position and output and can reproduce positional information with < 2 mm precision. Future work will involve screen heterogeneity corrections, improved transformation methods to increase large-field accuracy at the screen edges, and development of a phantom with this technology for routine daily, weekly and monthly MR-linac QA.
Funding Support, Disclosures, and Conflict of Interest: This project has been sponsored by National Institutes of Health research grants R01EB023909 and R44CA199681. The authors acknowledge the Irradiation Shared Resource at the Norris Cotton Cancer Center at Dartmouth with NCI Cancer Center Support Grant 5P30 CA023108-41. Brian W Pogue reports financial interest in DoseOptics LLC.