Analysis of a surface imaging system using a six degree-of-freedom couch

Purpose To validate surface imaging (SI)-reported offsets using a six degree-of-freedom couch and an anthropomorphic phantom for commissioning and routine quality assurance of an SI system used for stereotactic radiosurgery (SRS). Methods An anthropomorphic phantom with a radiopaque ball bearing (BB) placed either anterior, midline, or posterior, was tracked with SI with a typical SRS region of interest. 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 degrees, 45 degrees, 90 degrees, 270 degrees, 315 degrees with megavolt (MV) images taken to account for couch walkout. The SI residual error (epsilon), the difference between SI reported offset and MV or trajectory log position, was calculated. Residual errors were measured with and without one SI pod blocked. Results The median [range] of magnitude of translational epsilon was 0.13 [0.07, 0.21], 0.16 [0.11, 0.26], 0.61 [0.50, 0.68], 0.49 [0.42, 0.55], 0.55 [0.38, 0.72] mm for couch rotations of 0 degrees, 45 degrees, 90 degrees, 270 degrees, 315 degrees, respectively, for the midline BB and no pod blocked. The range of all translational epsilon from all couch angles (with and without pod block) at different BB positions is [0.05, 0.96] mm. The absolute range of difference when changing BB position when no pod is blocked in median translational epsilon is [0.01, 0.40] mm with the maximum at BB posterior. The absolute range of difference when not changing BB positions with and without pod block in median translational epsilon is [0.01, 0.37] mm with the maximum at BB posterior and couch 315 degrees. Conclusion SI system and linac trajectory log analysis can be used to assess SI system performance with automated couch motion to validate SI accuracy.