Geometric and dosimetric effects of image co-registration workflows for Gamma Knife frameless radiosurgery

The Gamma Knife® Icon™ CBCT facilitates frameless radiosurgery. In the vendor-recommended workflow, MRI is co-registered directly to CBCT for planning. Alternatively, MRI is co-registered to a diagnostic CT, which is then co-registered to CBCT. Our objective is to evaluate if this additional CT is necessary for more accurate registrations. Nine small spherical targets were generated onto 14 patient data-sets. Single-shot treatment plans were created. Geometric and dosimetric differences between the two workflows were determined. Mean target displacement was 0.5±0.3mm; average PTV coverage loss was 4.3±5.0%. For 19 clinical targets in 14 patients, the mean displacement and coverage change was 0.6±0.4mm and 1.3±1.6%. Eleven surrogate landmarks were contoured on a phantom MRI and registered to the CBCT using both workflows. The registration uncertainty was 0.50±0.65mm and 0.32±0.47mm for the MRI-CT-CBCT and MRI-CBCT respectively. As neither workflow was significantly more accurate, the additional CT is unnecessary for most cases.