Author: Ting Chen, Siming Lu, Tanxia Qu, Hesheng Wang 👨🔬
Affiliation: NYU Langone Health 🌍
Purpose: This study, the first in a two-part series, aims to determine the isocenter position and size for the MR Linac UNITY, focusing on the central axis (CAX) alignment of its Multi-Leaf Collimator (MLC). The UNITY system comprises three subsystems: (1) the Source Subsystem (SS), which includes the MLC and JAW fixed on a non-rotational collimator mounted on an O-ring; (2) the Target Subsystem (TS), a Z-axis-only movable table; and (3) the Guide Subsystem (GS), featuring an MV EPID for quality assurance (QA) and an MRI for patient positioning and motion management. This work evaluates the SS’s (U, V) coordinates for the CAX and verifies vendor-provided data.
Methods: U values were measured across 12 gantry angles by detecting the outer edges of the MLC field using a MATLAB-based Canny edge detection algorithm. V values were determined by analyzing the edges of triangular patterns formed by MLC leaves, calculating cross points, and averaging the results.
Results: The mean measured [U, V] values were [512.5, 651.1], closely matching the vendor’s [511.3, 651.4], with a maximum discrepancy of 0.4 mm at specific gantry angles (300° and 120°). Minimal standard deviations (<0.1%) and consistency over time confirm the system's robustness. The EPID marker (‘+’) demonstrated precise alignment for QA instruments like the profiler.
Conclusion: Our independent measurements validate the vendor’s (U, V) values, confirming the stability and rigidity of the Elekta MR Linac UNITY’s Source Subsystem. The consistent positioning of the MLC relative to the EPID across gantry angles ensures reliable system performance.