Assessing CBCT-based patient positioning accuracy on the Gamma Knife IconTM via Presage® 3D absolute dosimetry

Abstract

Cone beam computed tomography (CBCT) imaging has been implemented on the Leksell Gamma Knife IconTM for repeated patient positioning in mask-based Gamma Knife radiosurgery. The purpose of this study was to evaluate the accuracy of the CBCT-based patient positioning on the Gamma Knife IconTM. Two Presage phantoms of 15 cm diameter and 10 cm height were irradiated with identical shot placements on an Acoustic Neuroma target with the same prescription dose following standard mask-based treatment workflow according to two different fraction schedules: a single fraction of 7.5 Gy and 5 fractions with 1.5 Gy fraction dose. On the top and the bottom portions of each phantom, 8 single 16 mm collimator shots were delivered with maximum doses from 2 Gy to 20 Gy for dose sensitivity calibration. The irradiated Presage phantoms were scanned and analyzed using an OCTOPOUS optical CT scanner. Both the absolute dose distributions and the relative dose distributions for the Acoustic target on each phantom were compared with those from the treatment planning system. The relative dose distribution from the single fraction irradiation agrees better with the planning system than the 5 fraction irradiation, indicating noticeable change in the dose distribution caused by the phantom positioning/repositioning process. No difference between the absolute dose distributions from the two phantoms could be identified because of the large uncertainty in the experiment data.