Analysis of a Surface Imaging System Using a Six Degree of Freedom Couch for Setting Stereotactic Radiosurgery Treatment Action Level
X Zhao1*, E Covington2, R Popple3, (1) University of Alabama at Birmingham, Birmingham, AL, (2) University of Michigan, Ann Arbor, MI, (3) University of Alabama at Birmingham, Birmingham, AL
Presentations
(Saturday, 3/26/2022) [Central Time (GMT-5)]
Purpose: To establish action levels for surface imaging (SI) during stereotactic radiosurgery (SRS).
Methods: An anthropomorphic phantom with a radiopaque ball bearing (BB) placed either anterior, midline or posterior, was tracked with SI with or without one SI camera pod blocked mimicking pod blockage by gantry rotation during SRS treatment. Automated couch motion in all six degrees of freedom was programmed and delivered on a linac. SI system logs were synchronized with linac trajectory logs. Ten random couch positions were selected at couch 0, 45, 90, 270, 315° with MV images taken to account for couch walkout. The SI residual error (e), the difference between SI reported offset and MV or trajectory log position, was calculated and used to set thresholds for repeat radiographic imaging of patients.
Results: Range of all translational e from all couch angles (with and without pod block) at different BB positions is [0.05, 0.96] mm. The mean difference of median e with and without pod blockage is 0.1mm while it is 0.15mm when BB position is moved from midline to anterior or posterior. Using this data, a SI threshold of 0.80 mm for radiographic imaging for patient re-positioning was determined for midline lesions. This allows for 0.30 mm of intrinsic SI uncertainty and patient movement of 0.50 mm. However, if the target is located more anteriorly or posteriorly to the skull or a pod blockage presents, the threshold is increased to 0.90-0.95 mm, which comes from the increased uncertainty of SI when the target position is off-centered in the skull or a pod is blocked.
Conclusion: SI system and linac trajectory log analysis can be used to assess SI system performance and set action levels for SI during SRS.
Funding Support, Disclosures, and Conflict of Interest: Varian Medical Systems
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