A high-precision, geometric and registration accuracy full-system test method for adaptive SRS, demonstrated on Gamma Knife® Icon™

Abstract

A novel full-system test (FST) phantom and method have been developed to demonstrate and quality assure the geometric accuracy of image co-registration and overall shot delivery in the context of SRS using Gamma Knife® Icon™. The method uses Vernier scale bars to achieve sub-voxel precision co-registration measurements and pin-located radiochromic films to determine overall shot delivery precision. Validation tests demonstrated that artificially applied registration errors of < 0.15 mm could be accurately detected and quantified. Cross-validation of full-system test results with the manufacturer standard focal precision test demonstrated that both approaches measure similar focal precision errors, to within < 0.1 mm, and that registration and focal precision components of the full-system geometric error can be successfully decoupled using our Vernier FST approach. CBCT co-registration errors were shown to be of comparable magnitude to the focal precision errors, demonstrating that CBCT registration based in-mask treatments can achieve sub-voxel geometric accuracy, rivalling traditional frame-based immobilisation. This full-system geometric test method and phantom design concept is in principle applicable to any SRS technique involving image co-registration.

Keywords: Gamma Knife; Icon; co-registration; cone beam CT; full-system test.

Figure 1

a) Phantom design showing Vernier scale bar positions (all measurements in mm) and film slot locations. b) Photograph of phantom as manufactured.

Figure 2

a) Detail of Vernier scale bar design. Metal bars (black) were precisely set into a polymethylmethacrylate substrate block (grey) which was subsequently inserted into a machined slot in the phantom (Fig. 1). b) Photograph of manufactured Vernier scale block.

Figure 3

Analysis of registration error using Vernier scale. The moving image (blue) is overlaid on the reference image (greyscale) and the Vernier bar pairs are observed to align six pairs from the central pair, indicating a 0.6 mm offset along the scale bar direction at this location, due to misregistration.

Figure 4

In one vertically aligned Vernier scale, a 0.3 mm offset was apparent on the fixed image only, resulting from slight manufacturing inaccuracy. The scale is aligned at a position between the 7th and 8th bar pairs (red arrow) rather than the 6th (central) bar pair (yellow lines). This resulted in a systematic error in all degrees of freedom, which was identified and removed from the results presented (see text and Figure 5)

Figure 5

Residual error distributions, following subtraction of applied errors from measured errors. Distributions were derived by a 4 observer intercomparison based on 6 randomly-generated synthetic misregistrations. a) Translational and b) rotational residuals prior to correction for systematic manufacturing inaccuracy. c) Translational and d) rotational residuals following systematic correction.