The effect of intra- and inter-fractional motion on target coverage and margins in proton therapy for uveal melanoma

<i>Introduction.</i>This study aims to assess the effective lateral margin requirements for target coverage in ocular proton therapy (OPT), considering the unique challenges posed by eye motion and hypofractionation. It specifically addresses the previously unaccounted-for uncertainty contribution of intra-fractional motion, in conjunction with setup uncertainties, on dosimetric determination of lateral margin requirements.<i>Method.</i>The methodology integrates dose calculations from the in-house developed treatment planning system OCULARIS with measured intra-fractional motion, patient models from EyePlan and Monte Carlo (MC) sampling of setup uncertainties. The study is conducted on 16 uveal melanoma patients previously treated in the OPTIS2 treatment room at the Paul Scherrer Institute (PSI).<i>Result.</i>The retrospective simulation analysis highlights a significant impact of non-systematic factors on lateral margin requirements in OPT. Simulations indicate that reducing the 2.5 mm clinical lateral margin, represented by a 2.1 mm margin in this work, would have resulted in inadequate target coverage for two patients, revealing a greater impact of non-systematic factors on lateral margin requirements.<i>Conclusions.</i>This work characterizes intra-fractional motion in 16 OPT patients and identifies limitations of clinical margin selection protocols for OPT applications. A novel framework was introduced to assess margin sufficiency for target coverage. The findings suggest that prior research underestimated non-systematic factors and overestimated systematic contributions to lateral margin components. This re-evaluation highlights the critical need to prioritize the management of non-systematic uncertainty contributions in OPT.