SU-F-T-640: Feasibility of Using a Commercially Available Surface Guided Radiotherapy System with An Open-Face SRS Immobilization System.

PURPOSE: To evaluate the inherent accuracy of using a surface guided radiotherapy system (SGRT) in the setup and monitoring of patients receiving stereotactic radiosurgery with an open-face SRS immobilization system.

METHODS: An anthropomorphic head phantom was set up using the Qfix Encompass SRS Immobilization System on a Varian Edge with OSMS and Varian TrueBeam with AlignRT. The phantom was positioned at 0° gantry and couch. A reference image was acquired using the SGRT system and an ROI was created over the mask opening. The couch and gantry were rotated to different combinations focusing on clinically used SRS gantry/couch combinations and those blocking the SGRT cameras. Perceived surface deviation by the SGRT system from the reference image was recorded. A Winston-Lutz test was performed on couch angles tested and used to exclude couch walkout. The deviation magnitude was calculated using translational values and rotational raw values were recorded.

RESULTS: The maximum couch walkouts were: 0.4mm (Edge) and 0.5mm (TB). Solely rotating the gantry resulted in a median couch deviation of 0.2mm and range of 0.1-0.3mm for both linacs. Only rotating the couch (0° gantry) resulted in median deviations of 0.6mm and 0.5mm with ranges of 0.3-1.0mm and 0.3-0.7mm for the Edge and TB, respectively. Combining gantry and couch rotations, the median deviations were 0.7mm and 0.9mm with ranges of 0.3-1.1mm and 0.2-1.9mm for the Edge and TB, respectively. Including all combinations, rotation, roll, and pitch median deviations ranged from 0.1-0.3° with pitch demonstrating consistently higher values and a maximum deviation of 1.0° (both linacs).

CONCLUSION: SGRT is a reliable monitoring tool, though taking into account system fluctuations, 1mm is too restrictive a site tolerance to use with the Qfix Encompass mask. Gantry rotation has little effect on system fluctuation even with camera blockage, whereas couch rotation has a larger effect.