Visual Isocenter Position Enhanced Review (VIPER): a Cherenkov imaging-based solution for MR-linac daily QA.

Abstract

This study demonstrates a robust Cherenkov imaging-based solution to MR-Linac daily QA, including mechanical-imaging-radiation isocenter coincidence verification. A fully enclosed acrylic cylindrical phantom was designed to be mountable to the existing jig, indexable to the treatment couch. An ABS plastic conical structure was fixed inside the phantom, held in place with 3D-printed spacers, and filled with water allowing for high edge contrast on MR imaging scans. Both a star shot plan and a four-angle sheet beam plan were delivered to the phantom; the former allowed for radiation isocenter localization in the x-z plane (A/P and L/R directions) relative to physical landmarks on the phantom, and the latter allowed for the longitudinal position of the sheet beam to be encoded as a ring of Cherenkov radiation emitted from the phantom, allowing for isocenter localization on the y-axis (S/I directions). A custom software application was developed to perform near-real-time analysis of the data by any clinical user. Calibration procedures show that linearity between longitudinal position and optical ring diameter is high (R2 >0.99), and that RMSE is low (0.184mm). The star shot analysis showed a minimum circle radius of 0.34mm. The final isocenter coincidence measurements in the lateral, longitudinal, and vertical directions were -0.61mm, 0.55mm, and -0.14mm, respectively, and the total 3D distance coincidence was 0.83mm, with each of these being below 2mm tolerance. This novel system provided an efficient, MR safe, all-in-one method for acquisition and near-real-time analysis of isocenter coincidence data. This represents a direct measurement of the 3D isocentricity. The combination of this phantom and the custom analysis application makes this solution readily clinically deployable after the longitudinal analysis of performance consistency.