Purpose: Stereotactic paraspinal treatment has become increasingly popular due to its favorable clinical outcome. An often-overlooked factor that compromises the effectiveness of such treatment is the patients’ involuntary intrafraction motion. This work introduces an in-house software that quantifies such motion for accurate motion monitoring.
Methods: The software requires a separate full-trajectory CBCT after patient setup to establish reference projections. Once treatment starts, the software reads in the intrafraction motion review (IMR) image acquired by TrueBeam and compares it against the corresponding reference projection to instantly determine the 2D shifts of the vertebrae being monitored. To evaluate its performance, an anthropomorphic phantom was shifted 5 mm in three orthogonal directions, separately and altogether, immediately after the full-trajectory CBCT but prior to treatment. For each scenario of shift, the same nine-angle fixed-gantry IMRT plan was delivered. At each angle, three IMR images were acquired sequentially every 200 MU. For each IMR image, the software-reported 2D shift was compared with ground truth. These tests were repeated with three degree of rotation, pitch, and roll, respectively.
Results: The maximum standard deviation of the software-reported shifts for each set of three IMR images was 0.16 mm, with 93rd percentile below 0.03 mm. For translational shift, the maximum registration error was 0.44 mm, with 93rd percentile below 0.26 mm. Left unaccounted for, rotation and pitch degraded the registration accuracy mainly in the longitudinal direction while roll degraded it mainly in the lateral direction. The maximum registration errors under rotation, pitch, and roll were 2.97, 1.44, 2.72 mm, respectively.
Conclusion: Our in-house software has high repeatability and accuracy in registering IMR images with the reference projections for motion monitoring. Rotation, pitch, and roll degrade registration accuracy and need to be accounted for in future work.
Funding Support, Disclosures, and Conflict of Interest: The presented study is partially supported by Varian Medical Systems Research Collaborations.