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Assessment of the dosimetric impact of intra-fraction motion during frameless treatment delivery on GammaKnife® Icon™
Hannah Taylor, Peter Fallows and Gavin Wright

This study investigated the impact of patient motion on the dosimetric quality of treatment plans for metastatic patients undergoing frameless GammaKnife® Icon™ treatments. By quantifying dosimetric robustness at increasing high definition motion management (HDMM) gating tolerances, this study investigated the possibility of increasing the HDMM threshold for patients treated at our centre from our current standard of 1 mm.

Methods: Motion was retrospectively simulated by shifting the stereotactic co-ordinates of shots in treatment plans using three motion models. Dosimetric quality indicators of original and shifted plans were compared. Influence of target location and size was determined.

Results: Motion models showed median (p-value) absolute changes in target coverage of up
to −0.133% (<0.0001), −0.267% (<0.0001) and −0.667% (<0.0001) for HDMM tolerances of 1mm, 1.5mm and 3mm. The greatest median (p-value) absolute changes in Paddick Conformity Index (PCI) and Gradient Index (GI) were −0.008 (0.0032) and 0.017 (0.6893). A reduction in target size correlated weakly with greater changes in target coverage for all models and HDMM tolerances (r2 =0.040–0.309). No location dependence was observed.

Conclusion: HDMM tolerances up to and including 3mm all resulted in negligible changes in PCI and GI. Target coverage exhibited greater sensitivity to motion, but only at 3mm was the target coverage reduced below local planning aims. Our HDMM tolerance could therefore potentially be increased to 1.5mm, with likely benefits to treatment delivery efficiency.

Keywords: GammaKnife, Icon, intra-fraction motion, gating, nose tracking, HDMM

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