ESTRO 2025 - Abstract Book

S4091

RTT - Patient care, preparation, immobilisation and IGRT verification protocols

ESTRO 2025

2552

Digital Poster Comparison of inter-observer variability in positioning using CBCT between single- and multi-isocenter treatments Pawel Drozd, Karen Villaquiran, Hubert Gabrys, Stephanie Tanadini-Lang, Matthias Guckenberger, Indira Madani, Ina Nilo, Sophie Perryck Klinik für Radio-Onkologie, Universitätspital Zürich, Zürich, Switzerland Purpose/Objective: The single-isocenter technique allows multiple targets to be treated simultaneously. This significantly reduces the treatment time, enhances patient comfort, and improves efficiency of the radiotherapy workflow. However, patient positioning based on position of all lesions instead of one may lead to a larger uncertainty in patient positioning. This study evaluates the inter-observer variability of positioning corrections in single-isocenter versus multi isocenter radiotherapy. Material/Methods: All nine patients with eleven treatment plans were treated within a randomized phase II study comparing single with multiple isocenter technique. Patient positioning variability was assessed as a secondary endpoint by two independent radiation therapists using translational shifts (lateral, longitudinal, and vertical) and rotations (yaw, pitch, and roll). Systematic differences in matching were analyzed with the Wilcoxon signed-rank test, comparing inter-observer variability for single-isocenter and multi-isocenter scenarios. Variability in matching accuracy was assessed using Levene’s test. Furthermore, the agreement between the observers was evaluated with the intraclass correlation coefficient (ICC).

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