S2943
RTT- RTT operational practice and workflow innovations
ESTRO 2026
Material/Methods: Between May and October 2024, pre-treatment CBCT images were acquired at the Gamma Knife Unit of the IRCCS San Raffaele Hospital for 350 patients treated with GK ICON for intracranial lesions. Of these, 310 in a single session and 40 with fractionated schedules (up to 5 sessions), maintaining the stereotactic helmet for the entire duration of the treatment. Pre-treatment CBCTs were co-registered with reference images, MRI planning for single-fraction treatments and CBCTs from the first session for fractionated treatments, to assess translational and rotational shifts. The number of dose recalculations by the clinician was also noted.
Conclusion: The results of this study indicate that discrepancies between planning images and pre-treatment CBCTs, although statistically significant, are not attributable to set-up errors. The most evident differences emerged in single-fraction treatments planned on MRI images, suggesting that the main source of error lies in geometric distortions of MRI imaging rather than in the set-up.The integration of pre-treatment CBCT into the Gamma Knife ICON is therefore confirmed as a fundamental support for set-up verification and control. Based on these results, the workflow has been updated by introducing pre-planning CBCT as a stereotactic reference, while MRI is now mainly used for lesion characterisation. References: Mendel JT, Schroeder S, Plitt A, et al. Expanded Radiosurgery Capabilities Utilizing Gamma Knife Icon™. Cureus. 2021;13(3):e13998. Published 2021 Mar 19. doi:10.7759/cureus.13998Nakazawa H, Mori Y, Komori M, et al. Validation of accuracy in image co- registration with computed tomography and magnetic resonance imaging in Gamma Knife radiosurgery. J Radiat Res. 2014;55(5):924-933. doi:10.1093/jrr/rru027Masitho S, Putz F, Mengling V, et
Results: In single-session treatments, the average translational and rotational shifts resulted in a maximum average shot deviation of 0.96 mm, mainly along the longitudinal axis. For this type of treatment, the analysis of target volume coverage (V100%) showed an average reduction from 99.9% in MRI planning (MRI- based stereotactic reference system) to 96.1% in pre- treatment CBCT (CBCT-based), a statistically significant difference (p < 0.005) confirmed by Student's t-Test and linear regressions. In fractionated treatments, with a reference system based on the CBCT of the first session, however, the average coverage decreased from 99.1% to 98.5% (p < 0.005) between fractions, indicating a minimal deviation.These differences do not appear to be attributable to set-up errors: in fractionated treatments, the variation is minimal, while in single sessions, based on MRI-CBCT comparison, the discrepancies are more pronounced.
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