Room: AAPM ePoster Library
Purpose: Open face masks offer increased exposure of the patient’s face for patient positioning at set up and motion management during treatment delivery using surface guided radiation therapy (SGRT). This study aims to quantify falsely generated positional shifts from the surface guided imaging system for patients immobilized with open face masks due to changes in facial expression.
Methods: Volunteers (n=10) were immobilized in open face masks and the SGRT facial region monitored was varied in size to represent different mask openings readily accessible to radiation oncology clinics. Peak isocenter shifts were recorded by the surface imaging system in the translational and angular directions as participants opened their eyes and changed facial expressions, without intentionally rotating their head. A relaxed facial expression with eyes closed was used as a baseline between experiments and the total isocenter vector shift was calculated. This was performed in both standard and SRS specific resolutions with the CRAD Catalyst HD system.
Results: The mean total isocenter vector shift observed due to changing facial expressions ranged from 1.42mm for SRS specific to 1.65mm for standard resolutions for all facial volumes analyzed. False positional corrections provided by the surface guided imaging system were amplified when smaller regions of interest were monitored. Shifts generated in SRS resolution had less variance when compared to standard resolution when participants changed facial expressions and opened their eyes.
Conclusion: Clinics that rely on SGRT to monitor intrafraction motion for patient’s immobilized with open face masks should analyze as much surface area of the exposed skin as possible to reduce tracking errors. A larger region of interest has been demonstrated to reduce the impact of false positional corrections provided by the surface imaging system. These potential errors should be considered in the treatment planning and delivery process.
Funding Support, Disclosures, and Conflict of Interest: This research was funded in part by a vendor grant.