Initial experience using IDENTIFY for motion monitoring during frameless VMAT radiosurgery.

Purpose: We report our initial clinical experience using IDENTIFY, an optical surface guidance system, for monitoring intra-fraction motion during stereotactic radiosurgery with HyperArcTM. Methods: The IDENTIFY system consists of three ceiling mounted camera pods. For each treatment fraction, a reference surface was captured after radiographic image guidance prior to the first beam on. Surface guidance and linear accelerator trajectory logs were used to evaluate the reported offsets relative to the reference surface. Offsets were obtained at the start of treatment (after radiographic image alignment), end of treatment, and during treatment at 4 gantry angles: 140 and 220 degrees, when all three camera pods had a clear view, and 50 and 310 degrees, when the view of one of the camera pods was obstructed. Results: 733 fractions of 295 treatment plans were evaluated. The average treatment time was 3.35 min (range 1.87 to 7.45 min). The mean (x,y,z) translation offset at the end of treatment was (0.00, -0.01, -0.07) mm, with median magnitude 0.27 mm and 95% of magnitudes were less than 0.97 mm. The mean reported offset during treatment was (-0.40, 0.23, -0.08), (-0.25, 0.02, -0.16), and (-0.16, 0.21, -0.05) mm at table angles 45, 90, and 315 degrees, respectively. The mean change in translation when a camera pod was blocked by the gantry was (0.01, 0.14, 0.02), (0.10, 0.23, 0.07), and (-0.02, 0.15, -0.00) mm at table angles 45, 90, and 315 degrees, respectively. The fraction of offsets exceeding 1 mm was 4.4%, 6.9%, and 2.5% when all three cameras had an unobstructed view, and was 13.4%, 1.9%, and 10.6% when a camera pod was blocked at table angles 45, 90, and 315 degrees, respectively. Conclusion: The IDENTIFY system can be used to monitor for intra-fraction motion with submillimeter accuracy; however, the accuracy is reduced when a camera is obstructed by the gantry and/or the couch is rotated. Action levels should be established accordingly to account for this behavior. [ABSTRACT FROM AUTHOR]