S2210
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
2022;49(3):1814-1821. doi:10.1002/mp.15472 Keywords: Larynx, SBRT, MRI-Guided
surrogate points closest to and ±1cm superior/inferior to the larynx for each cine-MRI video frame. The magnitude, frequency and duration of beam- interrupting events was quantified. The GTV motion and skin surface surrogate point correlation was quantified using the Pearson correlation coefficient. Results: The mean BIDE beam-off time, frequency, duration and magnitude was 10±9%, 3.9±3.0 events/min, 1.2±1.1 seconds and 0.9±1.0 cm, respectively. The BIDE beam-off time exceeded 10% in 7/28 fractions and there was at least one BIDE with magnitude greater than 1.0 cm in 18/25 fractions. During beam interruptions, there was a weak statistically significant positive correlation (r=0.25, p=0.001) between the skin surrogate and GTV positions. Conclusion: Larynx motion analysis of cine-MRI during SBRT demonstrated intra- and inter-fraction variability of internal-motion-induced beam-interruption events. As evidenced by real-time larynx tracking, MRI guidance may offer advantages for larynx motion management during treatment. Given the unpredictability, frequency and magnitude of BIDEs, this analysis could inform PTV margin expansion for non-MR-guided laryngeal radiotherapy.
Digital Poster 894
Do rotational couch corrections induce unintended patient motion? A quantitative study of thoracic IGRT Tommy Andreasson, Sebastian Sarudis Department of Oncology, Länsjukhuset Ryhov, Jönköping, Sweden Purpose/Objective: Six degrees of freedom (6DOF) couches enable correction of both translational and rotational setup deviations in image-guided radiotherapy (IGRT). While these corrections improve geometric alignment, concerns remain as to whether applying rotational adjustments may induce secondary, unintended patient motion. This study aimed to evaluate whether rotational couch corrections increase post-adjustment setup uncertainty in thoracic radiotherapy. Material/Methods: A total of 120 thoracic patients (1,027 image pairs) treated on a Varian TrueBeam™ equipped with an Exact™ 6DOF couch were retrospectively analysed. All patients were immobilised in a supine position using a WingSTEP™ device for upper-body fixation. Three groups were defined: (1) control (no correction), (2) translation-only correction, and (3) full 6DOF correction. Two orthogonal kV image pairs were acquired per fraction: one for setup correction and one for verification.Residual displacements in the vertical, longitudinal, lateral, pitch, roll, and yaw directions were extracted from the verification images. Group-wise differences in 3D translation vectors were analysed using the Kruskal–Wallis test with Dunn’s post-hoc comparisons, and correlations between applied and residual parameters were assessed using Pearson’s r. Results: The initial alignments were comparable across groups. The median residual 3D translation vector was 0.14 cm across all groups, with no significant intergroup differences (p = 0.68).
References: 1.
Image Guided Radiation Therapy with MRI for the Treatment of Cancer, CONFIRM Study - NCI. June 23, 2016. Accessed November 10, 2025. https://www.cancer.gov/research/participate/clinical- trials-search/v?id=NCI-2020-045712. Huynh E, Boyle S, Campbell J, et al. Technical note: Toward implementation of MR-guided radiation therapy for laryngeal cancer with healthy volunteer imaging and a custom MR-CT larynx phantom. Med Phys.
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