(Saturday, 3/30/2019)
Room: Exhibit Hall
Purpose: We investigate the geometric and dosimetric differences between two registration workflows for the Leksell GammaKnife® Icon™: registering the planning MRI image directly to the on-board CBCT for stereotactic definition per the vendor recommendation, and an alternate workflow where the MRI is registered to a diagnostic CT, which is then in turn registered to the stereotactic CBCT. We aim to identify situations which are more susceptible to these differences.
Methods: For five patients, eight 4mm diameter and one 0.14mm diameter spherical lesions were contoured on the 3D T1 MRI scan. For each patient, the MRI was registered to the diagnostic CT, which was in turn registered to the CBCT. A single-shot plan was created; shot location and prescription isodose were selected to maximize conformity index while maintaining 100% PTV coverage. The MRI was then fused directly to the CBCT, and the plan was copied to the new registration. All registrations were done using GammaPlan®’s automatic registration process using a volume of interest encompassing the entire CBCT volume (whole skull). Geometric differences between the target center locations and the resulting changes in PTV coverage were recorded.
Results: The mean 3D displacement of target centers between registrations was 0.4±0.2mm (max=0.9mm). The mean decrease in target coverage was 3.0±2.4% (max=10%). The targets exhibiting largest displacements and most coverage loss were the two most posteriorly-located. The patient with the largest displacements (0.6±0.2mm) and most coverage loss (5.0±2.9%) had limited shoulder clearance which prevented the CBCT from capturing the base of skull region, potentially reducing the registration accuracy.
Conclusion: Geometric and dosimetric differences exist between the two registration workflows. Preliminary data indicate patients with posteriorly located targets or limited CBCT FOV may be more susceptible to CBCT-to-MR registration uncertainties.
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