Multi-institutional evaluation of isocentricity on a ring-gantry linear accelerator platform with double-stacked MLCs

This study aimed to characterize the isocentricity of ring-gantry linacs and evaluate variability across institutions and software. Isocentricity is assessed using vendor-provided and user-defined Winston-Lutz (WL) tests.

WL was performed at two institutions on a ring-gantry linac using a custom phantom over 1.5 years. Distal and proximal MLCs were tested, with measurements obtained before and after applying 3D shifts. WL analysis was performed using in-house, commercial (C1-C3), and open-source software. Assessed parameters included the 2D ball bearing (BB)-radiation offset, the 3D offset minimizing 2D errors, and 3D isocenter size. 3D isocenter size was additionally computed using gantry-only fields to remove collimator walkout contributions.

Results across institutions showed sub-millimeter agreement among software except for C1 (max 0.99 mm difference). Machine-dependent variations were observed (p < 0.03). Proximal and distal banks demonstrated small but statistically significant differences (p < 0.03) in 2D/3D offsets and isocenter size, with greater walkout in the proximal bank and poorer axis alignment in the distal bank. Collimator-walkout-corrected 3D isocenter size was not statistically different between proximal and distal bank or between pre- and post-3D-offset shift.

Isocentricity measurements depended on software, MLC bank, pre-/post-offset correction, and institution. For the tested machines, which showed a mean imaging-to-radiation isocenter coincidence of 0.8 mm and a radiation-isocenter size of 0.7 mm, a minimum targeting margin of 1.5 mm would be appropriate to account for geometric uncertainty. With the results from this study, clinicians can choose their own margins based on their machine characteristics.