Accuracy of the catalyst surface guidance system for patient monitoring during cranial SRS treatments

The use of surface guided imaging in cranial stereotactic radiotherapy provides a non-ionising form of patient position verification that gives information on when patient position errors may require corrections to be applied. This work assessed the accuracy of Catalyst+ HD system for treatment geometries commonly used in cranial SRS. Average Catalyst reported error as a function of couch rotation agreed with measured kV and MV walkout within 0.5 mm for the lateral and longitudinal directions. Change in Catalyst reported error with isocentre depth relative to the monitoring region of interest (ROI) from the surface was assessed and showed variation above 0.5 mm, but within 1 mm for isocentre depths ranging from 3 to 15 cm from the phantom surface. Gantry occlusion of Catalyst cameras caused a shift in reported position error that was also dependent on isocentre depth relative to the monitoring ROI. SRS MapCHECK patients specific QA results showed improvement in gamma passing rates for a workflow where Catalyst reported errors above 0.5 mm were corrected.