Author: Tanxia Qu, Hesheng Wang 👨🔬
Affiliation: NYU Langone Health 🌍
Purpose:
This study, the second in a two-part series, evaluates the isocenter position (the point in 3D space minimizing the sum of Euclidean distances to the MLC's central axis) and isocenter size (the largest distance) for the MR Linac UNITY. The isocenter, determined by the Source Subsystem (SS), serves as a reference point for calibrating the Guide Subsystem (GS) and targeting movement. Isocenter size is critical for calculating the CTV-PTV margin, alongside GS accuracy and target motion during treatment. Real-time motion management is enabled by MR Linac.
Methods: We used a stationary BB as the origin of a 3D XYZ coordinate system, independent of gantry angle. Two cases were evaluated: four cardinal gantry angles and 12 gantry angles spaced at 30° intervals. For case 1, the isocenter in the XY plane was the center of a box formed by the CAX lines, validating an in-house MATLAB algorithm. For case 2, no assumptions were made about the CAX origin.
Results:
In case 1 (four angles), the isocenter position was (0.8, -0.7, 0.6) mm, with a size of 0.4 mm. In case 2 (12 angles), the isocenter position was (0.8, -0.3, 0.7) mm, with a size of 0.7 mm.
Conclusion:
The results confirm vendor-provided parameters, showing that isocenter size is determined solely by the SS. For a rotatable table, BB movement may alter the isocenter size. Future work will assess isocenter size dependency on 36 or 72 gantry angles.