ИСТИНА

Leksell Gamma Knife Icon (Elekta AB) includes a cone-beam CT (CBCT) to define the stereotactic space without the need for an invasive frame. The aim of this study was to analyze the differences between the stereotactic frame - based coordinates and the CBCT - determined coordinates. We performed CBCT before frame-based stereotactic Gamma Knife radiosurgery for 212 patients as an additional quality assurance (QA) procedure within radiosurgery treatment. The rotational and translational shifts, maximum shot displacement,delivered maximum doses for critical structures and coverage of the tumors were recorded. The factors investigated were z-coordinates of the right and left posterior pins, tumor localization, Leksell stereotactic coordinates of the tumor, tumor volume, mean and maximum definition errors for the MR study. The statistical analysis was performed by the R statistical package. The maximum shot displacement in anatomy was more than 1 mm for 32 patients. Planned tumor coverage was no less than 99% but the delivered one was less 95% in 12 cases. The z-coordinate of the tumor (p=0.036), volume of the tumor (p=0.045) were associated with differences of the coverage. The x-coordinate of the tumor (p=0.029) and the mean definition error (p=0.024) on MR images were associated with maximum shot displacement. The understanding of the differences between the stereotactic frame - based and the CBCT 3D stereotactic space is an important aspect of Gamma Knife Icon radiosurgery. The differences between radiological and mechanical isocenters in case of frame-based radiosurgery are often assumed to be 0.2 - 0.5 mm. But the uncertainty of target and the structure localization could lead to more than 5% reduction of the prescribed dose coverage of the tumor. The method of stereotactic space definition (frame or CBCT) that obtains more accurate results should be determined as well as the clinical significance of the demonstrated shifts are supposed to be defined