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

Abstract

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°, 45°, 90°, 270°, 315° with megavolt (MV) images taken to account for couch walkout. The SI residual error (ε), 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 ε 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°, 45°, 90°, 270°, 315°, respectively, for the midline BB and no pod blocked. The range of all translational ε 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 ε 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 ε is [0.01, 0.37] mm with the maximum at BB posterior and couch 315°.

Conclusion: SI system and linac trajectory log analysis can be used to assess SI system performance with automated couch motion to validate SI accuracy.

Keywords: six degree-of-freedom couch; stereotactic radiosurgery; surface imaging; surface-guided radiotherapy.

FIGURE 1

(Left) Setup of anthropomorphic Styrofoam phantom with ball bearing (BB) placed at anterior, midline, and posterior of the phantom, with the same tracking region of interest (ROI) contoured in blue (right)

FIGURE 2

Comparison of phantom position reported by linear accelerator trajectory logs, surface imaging logs, and megavolt (MV) image analysis at couch 0° with midline ball bearing (BB) position and no pod block. When the couch angle, BB position or pod blocking were different, a similar pattern was observed. Here, the couch moved to 10 preprogrammed randomly chosen locations within a range of ±1.5 mm and ±0.5°. The same 10 preprogrammed couch positions were repeated for different couch angle, BB position or pod blocking scenarios. There are 11 blue dots indicated by MV images because the first position is always neutral for any couch angle

FIGURE 3

Surface imaging (SI) residual error for no pod block mean (red dot) and standard deviation (red error bar) of vertical, longitudinal, lateral directions in mm, roll, pitch and rotation in degree is shown for couch angles 90°, 45°, 0°, 315°, 270°. Left pod block data are shown in green diamonds, and right pod block data are shown in blue squares. Longitudinal and lateral residual errors were analyzed from IDENTIFY logs and megavolt (MV) imaging data, whereas the other four degrees were from IDENTIFY logs and linac trajectory logs

FIGURE 4

Median (red dots) and range (blue error bar) of magnitude of translation residual error $|{\epsilon }^{\mathrm{mid},\mathrm{pod}}(\theta )|$ (mm), from all 50 random couch positions from five couch angles at ball bearing (BB) position midline with no pod block (left), with left pod block (middle), and with right pod block (right). The scale of the plots is shown in the middle figure

FIGURE 5

Residual error of longitudinal direction (left) and lateral direction (right) in mm of IDENTIFY compared to linac trajectory log (green) and megavolt (MV) imaging (red) at ball bearing (BB) position midline with no pod block, where MV imaging showed comparable result with IDENTIFY overall. The scale of plots is shown in the left figure