S2258
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
Conclusion: For regular breathing, 6-second HS-CBCT captures full target motion. In slow and irregular breathing, however, localization and volumetric discrepancies may occur. In these scenarios, adaptive protocols tailored to patient-specific breathing periods and irregularities may help to ensure optimal trade-off between ITV definition and overall image quality. References: 1. Robar, J. L. et al. Novel Technology Allowing Cone Beam Computed Tomography in 6 Seconds: A Patient Study of Comparative Image Quality. Practical Radiation Oncology 14, 277–286 (2024).2. Bogowicz, M.
analyzed. The gross tumor volume (GTV) was delineated daily on MRI, with a 3-mm planning target volume (PTV) margin. All treatments were monitored using the CMM system with the Volumetric Overlap Criterion (VOICE), which quantifies the percentage of PTV volume within the three- dimensional gating boundaries, which was coincident with the PTV. The beam was automatically switched off when VOICE fell below 95%. Baseline shifts using CMM intrafraction drift correction were applied when the target showed a clear positional drift and started to remain persistently under the established threshold. Audit log files provided time-stamped 3D motion and beam-on data to derive total monitoring time, beam-on duration and ratio, maximum displacement amplitude, and intrafraction variability (standard deviation of 3D motion). Descriptive statistics were computed per patient and per fraction, and cumulative VOICE distributions were used to evaluate target coverage stability during beam
et al. Evaluation of a cone-beam computed tomography system calibrated for accurate
radiotherapy dose calculation. Physics and Imaging in Radiation Oncology 29, 100566 (2024).3. Lustermans, D. et al. Development of a novel 3D-printed dynamic anthropomorphic thorax phantom for evaluation of four-dimensional computed tomography. Physics and Imaging in Radiation Oncology 32, 100656 (2024). Keywords: HyperSight CBCT, Respiratory motion, Delineation Digital Poster Highlight 3792 Intrafraction motion and gating efficiency in hepatic MR-guided SBRT (MRIgSBRT) Paulo Ferreira 1 , Zelda Paquier 1 , Sara Poeta 1 , Ana Margarida Costa 2 , João Perdigão 2 , Leonor Saavedra 2 , Nicolas Jullian 2 , Christelle Bouchart 2,3 , Akos Gulyban 1 , Robbe Van den Begin 2 1 Medical Physics Department, Institut Jules Bordet, Hopital Universitaire de Bruxelles, Université Libre de Bruxelles, Brussels, Belgium. 2 Radiation Oncology Department, Institut Jules Bordet, Hopital Universitaire de Bruxelles, Université Libre de Bruxelles, Brussels, Belgium. 3 Laboratory of Experimental Gastroenterology, Université Libre de Bruxelles, Route de Lennik 808, 1070 Brussels, Belgium Purpose/Objective: Liver tumors exhibit substantial respiratory-induced motion during radiotherapy, making real-time motion management essential for accurate dose delivery. The Comprehensive Motion Management (CMM) system of the Elekta Unity MRI-Linac enables continuous target tracking and beam gating, irradiating only when the target remains within predefined boundaries. This study retrospectively quantified intrafraction motion and treatment efficiency in liver MRI-guided stereotatic radiotherapy (MRIgSBRT) under abdominal compression. Material/Methods: Twenty-five liver patients (140 fractions) treated between September 2024 and October 2025 were
delivery. Results:
A total of 140 fractions were analyzed. The mean total monitoring duration was 16.0 min (range 3.0–50.3), with a mean beam-on time of 11.2 min, corresponding to a mean beam-on ratio of about 62%. Baseline shifts were observed in 40 of 140 fractions (29%), representing CMM drift-correction events occurring at some point during treatment. Across all fractions, the maximum 3-D motion amplitude had a mean of 14 mm (range 4–57 mm), with most displacements between 10 and 20 mm. The average intra-fraction variability of motion was approximately 3 mm (SD), indicating large but repeatable respiratory motion. The cumulative VOICE distribution showed that the target remained within the gating tolerance for roughly 85– 90% of total irradiation time, confirming effective motion management throughout beam delivery.
Figure 1: Beam-On ratio and maximum 3D displacement per patient and fraction.
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