The potential of an optical surface tracking system in non-coplanar single isocenter treatments of multiple brain metastases.

To evaluate the accuracy of a commercial optical surface tracking (OST) system and to demonstrate how it can be implemented to monitor patient positioning during non-coplanar single isocenter stereotactic treatments of brain metastases. A 3-camera OST system was used (Catalyst HD™, C-RAD) on a TruebeamSTx with a 6DoF couch. The setup accuracy and agreement between the OST system, and CBCT and kV-MV imaging at couch angles 0° and 270°, respectively, were examined. Film measurements at 3 depths in the Rando-Alderson phantom were performed using a single isocenter non-coplanar VMAT plan containing 4 brain lesions. Setup of the phantom was performed with CBCT at couch 0° and subsequently monitored by OST at other couch angles. Setup data for 7 volunteers were collected to evaluate the accuracy and reproducibility of the OST system at couch angles 0°, 45°, 90°, 315°, and 270°. These results were also correlated to the couch rotation offsets obtained by a Winston-Lutz (WL) test. The Rando-Alderson phantom, as well as volunteers, were fixated using open face masks (Orfit). For repeated tests with the Rando-Alderson phantom, deviations between rotational and translational isocenter corrections for CBCT and OST systems are always within 0.2° (pitch, roll, yaw), and 0.1mm and 0.5mm (longitudinal, lateral, vertical) for couch positions 0° and 270°, respectively. Dose deviations between the film and TPS doses in the center of the 4 lesions were -1.2%, -0.1%, -0.0%, and -1.9%. Local gamma evaluation criteria of 2%/2 mm and 3%/1 mm yielded pass rates of 99.2%, 99.2%, 98.6%, 89.9% and 98.8%, 97.5%, 81.7%, 78.1% for the 4 lesions. Regarding the volunteers, the mean translational and rotational isocenter shift values were (0.24 ± 0.09) mm and (0.15 ± 0.07) degrees. Largest isocenter shifts were found for couch angles 45˚ and 90˚, confirmed by WL couch rotation offsets. Patient monitoring during non-coplanar VMAT treatments of brain metastases is feasible with submillimeter accuracy.