Room: Exhibit Hall | Forum 5
Purpose: To verify the feasibility of using the MV portal imager to monitor intra-fractional motion and compare to the performance of the Optical Surface Monitoring System (OSMS) (Varian Medical Systems, Palo Alto, CA)
Methods: A skull phantom was placed in an open-faced thermoplastic mask and a CT scan obtained. A treatment plan was created and the phantom setup for treatment. Intra-fractional motion was simulated via couch shifts. MV images and OSMS real-time-deltas (RTDs) were acquired at couch angles of 0Â°, 45Â°, 315Â°, and 90Â° for no simulated shift and Â±1 mm in the vertical, longitudinal, and lateral directions. Automatic registration was used to determine the MV image shifts, which were compared to the RTDs detected by OSMS.
Results: The MV image shifts in the lateral/longitudinal directions agreed to within 0.2 mm at all table angles; however, the MV images failed to detect the vertical shift and always reported 0 mm. The OSMS vertical and lateral RTDs were both within 0.1 mm. However, at table angles Â±45Â° and no applied shift, the longitudinal RTDs displayed a systemic offset of 1.1 mm. For Â±1 mm applied longitudinal shift, the RTD systemic offset was 0.4 mm. The lateral RTDs were within 0.2 mm at Â±45Â°.
Conclusion: The MV portal imager and OSMS have comparable accuracy in detecting shifts in the lateral direction. Only OSMS was able to detect vertical motion, but displayed a systematic longitudinal offset for table angles Â±45Â°. Additional work is necessary to characterize the OSMS offset for non-zero table angles. Both systems can be used in tandem to ensure accurate shift detection for all directions.
Funding Support, Disclosures, and Conflict of Interest: UAB receives research funding from Varian Medical Systems. R Popple has received speaking honoraria from Varian Medical Systems.