Purpose: With the implementation of template-based stereotactic radiosurgery utilizing non-coplanar volumetrically modulated arc therapy (n-VMAT), it is hypothesized that the traditional Winston-Lutz (WL) test may no longer be sufficient to represent isocenter deviation. Triggered MV imaging at specified angular increments throughout a treatment arc could be used to autonomously evaluate isocenter deviation for a given template’s geometry.
Methods: A 2.5 mm ball-bearing WL phantom was aligned to mechanical isocenter and then kV isocenter to represent a patient setup verification on two Truebeam Linacs. 2 cm MLC defined square aperture images were taken at 5-degree increments for 3 arcs used in a clinical fractionated SRS template and exported to an in-house MATLAB script. 8 conventional static WL images were taken for comparison. For each arc, the isocenter deviation was extracted and plotted as a function of gantry angle and the maximum deviation found in each direction. Potential plan deviations were evaluated by applying the ‘worst-case’ deviations to each arc in a series of clinically treated SRS plans using identical geometry.
Results: The average maximum deviation was found to be larger for the treatment arc WL than shown in the convention WL test (0.75 mm vs 0.49 mm) but still within vendor specification. After applying the maximum isocenter deviations for each arc, target coverage dropped on average for both multi-lesions single-isocenter and single-lesion plans from 99.3 ± 0.45% to 97.7 ± 1.2%, maximum loss of 2.72%. Conformity index for each lesion dropped by 0.023 on average, maximum of 0.0472, whilst there were no statistically significant changes on dose to critical organs including normal brain.
Conclusion: We have demonstrated a novel non-coplanar arc-based WL test, via triggered imaging, that better characterizes isocenter wobble for arc-based stereotactic treatments than the traditional static WL test. This method would be specifically useful for single-isocenter/multi-lesions SRS setting.