Mitigating ocular torsion induced margin loss in ocular proton therapy via collimator rotation

<i>Purpose.</i>Ocular torsion is a challenge occasionally encountered in ocular proton therapy (OPT) consisting of a rotation of the eye about the visual axis. This can result in the safety margin being compromised and reduced conformity of the dose field to the target. This note investigates the effect of ocular torsion on the lateral margin to verify and explore quantitative adaptation strategies to mitigate the adverse effect on this margin.<i>Methods.</i>OCULARIS, an in-house OPT research planning tool, was used to simulate 14 patients undergoing OPT. The lateral margin was determined for each patient at ocular torsion angles ranging from -8^∘to 8^∘in discrete steps of 2^∘, with 19 collimator rotations simulated at each torsion angle.<i>Results.</i>Margin loss increases with greater ocular torsion, with significant inter-patient variability being influenced by the shape of the target. Aligning collimator rotation with ocular torsion nominal torsion matching (NTM) retains 61% of the margin, patient-specific adaptations achieve superior dose conformity to the target. A simple regression method, setting the collimator rotation to the ocular torsion angle minus 1^∘for torsions greater than 2^∘, offers some benefit over NTM in this cohort.<i>Conclusions.</i>Margin loss increases with ocular torsion, with the extent of loss being influenced by patient-specific geometry. The NTM collimator rotation strategy was found to adequately compensate for torsion-induced margin loss. Alternative collimator rotation strategies were also explored, including a framework for optimising collimator rotation in the event of ocular torsion.