Purpose: Treatment modalities such as Varian’s HyperArc allow convenient and efficient treatment of multiple stereotactic targets in a single isocenter. Confirming geometric accuracy of the mechanical, radiation, and imaging systems for off-axis targets is crucial for these treatments, but centering an MLC field on an off-axis target poses a significant challenge due to geometric offsets in the source-target direction and fixed MLC leaf width. While methods have been proposed to perform Winston-Lutz testing of off-axis targets relative to baseline measurements, this work aims to provide a simple solution for multi-target Winston-Lutz testing that is absolute, efficient, and reproducible.
Methods: The VisionRT Cube phantom was used, which includes five 7.5mm ceramic spheres, including 4 targets offset in three dimensions from a central target. Total distances vary from 2.3cm to 4.1cm. Eight fields were planned which included gantry, collimator, and table rotations. Within each field, 1.5cm MLC openings were centered on three targets including the central target. Exact MLC locations for centering on each target in the leaf motion direction, as well as known target offsets in the interleaf direction, were calculated from simple beam divergence projections. These known offsets were then subtracted from the Winston-Lutz results for each field. This test was repeated on five occasions to confirm reproducibility.
Results: Initial results showed a maximum isocenter offset of 1.35mm. After applying offset corrections, all targets for all fields showed accurate results with the largest offset being 1.03mm. The average offsets ranged from 0.32mm to 0.54mm for the various targets, while standard deviations ranged from 0.15mm to 0.21mm.
Conclusion: This proposed multi-target Winston-Lutz method provides an accurate and reproducible means to characterize off-center targeting performance. The process can be utilized during commissioning of a single isocenter multi-target SRS program and is simple enough to perform routinely.