Real-Time Eye Tracking for Non-Coplanar Linac-Based Ocular SRS/SRT Via a Non-Invasive, 3D Printed Adjustable Table-Mounted Device
Abstract
Purpose
External beam radiotherapy (RT) for ocular tumors is challenged by intra-fractional ocular motion. Although thermoplastic immobilization masks reduce head mobility, eye mobility remains problematic. Current motion-management options, including COMS brachytherapy, retrobulbar anesthesia, or surgically implanted radiopaque markers, increase invasiveness, treatment complexity, and limit patient accessibility. Camera-based eye tracking is a promising alternative; however, no real-time, noninvasive system compatible with the Encompass™ immobilization system and the extreme non-coplanar geometries of the HyperArc module has been reported. As part of establishing an external beam ocular stereotactic radiosurgery/radiotherapy (SRS/SRT) program at our institution, this study aimed to develop a noninvasive real-time eye-tracking system.
Methods
A CT scan of the Encompass™ fixation mask was used to design an adjustable eye-tracking frame in Fusion360, incorporating multiple independent adjustment points to maximize patient compatibility and reproducibility. The system consisted of a gaze-fixation LED, two infrared LEDs, and a single camera. Eye tracking was implemented using the pupil-center corneal reflection (PCCR) method. System performance was evaluated in six healthy volunteers using bilateral eye tracking. Volunteers fixated on the gaze LED for one minute, during which 60 consecutive frames were averaged to define a reference optic axis orientation. Gaze stability was assessed using a 5° conical threshold, and median angular deviations were measured.
Results
The adjustable device was fully compatible with the Encompass™ support system and HyperArc non-coplanar geometries, allowing independent adjustment of gaze-fixation and camera components without gantry interference. All volunteers successfully maintained gaze fixation, with a median angular deviation of 0.89° (range: 0.48–2.72°).
Conclusion
A noninvasive, adjustable eye-tracking system for real-time ocular motion management during LINAC-based treatments was successfully developed. This 3D-printed device and tracking algorithm enable a reliable method of intra-fractional motion management for external beam based ocular SRS/SRT. Further calibration to enable absolute tumor position derivation and clinical implementation is ongoing.