Gaze Stability During Ocular Proton Therapy: Quantitative Evaluation Based on Eye Surface Surveillance Videos

Abstract

Ocular proton therapy (OPT) is acknowledged as a therapeutic option for the treatment of ocular melanomas. OPT clinical workflow is deeply based on x-ray image guidance procedures, both for treatment planning and patient setup verification purposes. An optimized eye orientation relative to the proton beam axis is determined during treatment planning and it is reproduced during treatment by focusing the patient gaze on a fixation light conveniently positioned in space. Treatment geometry verification is routinely performed through stereoscopic radiographic images while real time patient gaze reproducibility is qualitatively monitored by visual control of eye surface images acquired by dedicated optical cameras. We described an approach to quantitatively evaluate the stability of patients’ gaze direction over an OPT treatment course at the National Centre of Oncological Hadrontherapy (Centro Nazionale di Adroterapia Oncologica, CNAO, Pavia, Italy).Pupil automatic segmentation procedure was implemented on eye surveillance videos of five patients recorded during OPT. Automatic pupil detection performance was benchmarked against manual pupil contours of four different clinical operators. Stability of patients’ gaze direction was quantified. 2D distances were expressed as percentage of the reference pupil radius.Valuable approximation between circular fitting and manual contours was observed. Inter-operator manual contours 2D distances were in median (interquartile range) 3.3% (3.6%) of the of the reference pupil radius. The median (interquartile range) of 2D distances between the automatic segmentations and the manual contours was 5.0% (5.3) of the of the reference pupil radius. Stability of gaze direction varied across patients with median values ranging between 6.6% and 16.5% of reference pupil radius.The measured pupil displacement on the camera field of view were clinically acceptable. Further developments are necessary to reach a real-time clip-less quantification of eye during OPT.