Download In phantom studies, Brainlab`s ExacTrac system has demonstrated

 In phantom studies, Brainlab’s ExacTrac system has demonstrated superb accuracies in detecting translational and rotational errors. However, it has been reported that when used on actual patients, the ExacTrac’s shift results can be of considerable errors, i.e., translational error can exceed 3 mm, and rotational error can be greater than 2 degree [ref 1]. Error of this magnitude is greater than what would be tolerated in a high precision spine radiosurgery procedure. Precaution was recommended when using ExacTrac for spine radiosurgery. Given the published data and recommendation, we implemented the ExacTrac guided spine treatment with caution. By incorporating CT‐on‐Rails imaging procedure, we added an independent 3D alignment verification to the ExacTrac system. This would make sure that the large errors in ExacTrac results, if there was, would be discovered before we treated the patients. It will increase our confidence in patient setup and treatment without significantly slow down the process. The long established in‐house CAT system was used to analyze the images for 3D patient alignment. The flow chart in fig 1 included CT‐on‐rail and CAT procedure. After the ExacTrac X‐ray correction was done, the BB’s were placed on the lasers cross on patient right, ant, and left side to define iso center on the daily CT images for CAT. The table position was recorded. After CT was acquired, the table was returned to the recorded position, and ExacTrac x‐ray verification was performed to have the final position for confirmation. The CAT analysis(fig 2) returned its corrected table position so that we can compared it with the ExacTrac determined treatment table position. Because that the iso center defined in the daily CT using CT‐on‐rail was determined by placing BB’s on the room laser, and because of the reproducible uncertainty in couch position when it is rotated 180 degree back and forth, it was reasonable to assume 1‐mm operational uncertainty in the defining iso center in daily CT. Therefore we will allow 2‐mm total discrepancy along each axis compared with ExacTrac results. The comparisons should be made on the final treatment table position data from the two methods. When the discrepancy is greater than 2 mm, careful analysis need be exercised to determine the cause of the problem and its significance. Fig 1. Flow diagram for the proposed spine SBRT Fig 2. CAT registration and correction [Reference]
1. Zheng Chang, et al., 6D image guidance for spinal non‐invasive stereotactic body radiation therapy: Comparison between ExacTrac X‐ray 6D with kilo‐voltage cone‐beam CT, Radiotherapy and Oncology Vol. 95, 116‐121, (2010)