S2885
RTT - RTT education, training, and advanced practice
ESTRO 2026
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 al. Accuracy of MRI-CT registration in brain stereotactic radiotherapy: Impact of MRI acquisition setup and registration method. Z Med Phys. 2022;32(4):477-487. doi:10.1016/j.zemedi.2022.04.004 Digital Poster 4228 Empowering RTTs: A lean synthetic CT-based plan- of-the-day feasibility study for prostate radiotherapy patients with bladder filling challenges Caitriona Kelly 1 , Claire Fitzpatrick 1 , Aodh MacGairbhith 1 , Sinead Brennan 1,2 , Elizabeth Forde 2,3 , Ciaran Malone 1,4 1 Radiation Oncology, St. Luke’s Radiation Oncology Network, Dublin, Ireland. 2 Trinity College Dublin, St James' cancer Institute, Dublin, Ireland. 3 Applied Radiation Therapy Trinity, Discipline of Radiation Therapy, Dublin, Ireland. 4 Erasmus MC Cancer Institute, University Medical Center Rotterdam, Rotterdam, Netherlands Purpose/Objective: Bladder filling variability in prostate radiotherapy risks exceeding OAR doses, repeated imaging, increased patient distress and prolonged visits. This study evaluates the feasibility and dosimetric impact of a vendor-neutral, RTT-led, plan-of-the-day (PoD) workflow using both CBCT - derived synthetic CTs (sCT) and AI - assisted contouring on non - adaptive Varian linacs, aiming to create a lean protocol that enables RTTs to manage anatomical variation consistently. Material/Methods: A retrospective review included 10 prostate cancer patients treated to 60 Gy in 20 fractions who had persistent low bladder volumes (bladder < planned for ≥ 3 fractions). Daily CBCTs prompting intervention using the bladder analysis task were converted to sCTs. MVision Contour+ auto - contoured OARs, these were reviewed and edited by RTTs. Rigid registration was used to map PTVs to the sCT. The clinical plan was recalculated in Eclipse v18 and evaluated against CHiPP trial bladder dose - volume constraints (DVCs). If bladder DVCs were exceeded, RTTs re - optimised the plan on the sCT to generate a PoD option. PoD bladder volumes were compared with subsequent fraction CBCTs to assess (1) PoD adoption potential and (2) achievable bladder dose reduction. Results: All on - treatment bladders modelled on sCT exceeded
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.
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. Keywords: Gamma Knife, Radiotherapy, Positioning 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,
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