ESTRO 2025 - Abstract Book

S3271

Physics - Intra-fraction motion management and real-time adaptive radiotherapy

ESTRO 2025

Conclusion: This study provides first evidence of a systematic gravity-induced tumour displacement during SABR lung treatments. A consistent dorsal tumour shift was observed while lateral and longitudinal movements remained random, and no patient specific parameters indicated an impact on the tumour shift. This previously unreported effect emphasizes the importance of pre- and post-treatment CBCT analysis for identifying significant intrafractional tumour motion. Further investigation of this effect could impact future SABR lung delivery strategies.

Keywords: Intrafractional Motion, SABR Lung, Gravity Effect

References: [1] Keall, P. J., Mageras, G. S., Balter, J. M., Emery, R. S., Forster, K. M., Jiang, S. B., ... & Yorke, E. (2006). The management of respiratory motion in radiation oncology report of AAPM Task Group 76 a. Medical physics , 33 (10), 3874-3900. https://doi.org/10.1118/1.2349696. [2] M. Brincker, H. Sand. (2024). Dorsal tumour shift in lung sbrt: impact of gravity, Physica Medica ,125 (1). https://doi.org/10.1016/j.ejmp.2024.103490.

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Mini-Oral Delineation uncertainties dominate over intrafraction motion in cone beam CT-based online adaptation of locally advanced rectal cancer (NCT05883800) Johanna Austrheim Hundvin 1,2 , Unn Hege Lilleøren 1 , Katrin Hämmerling 1 , John Alfred Brennsæter 1 , Inger Marie Løes 1 , Sara Pilskog 1,2 1 Department of Oncology and Medical Physics, Haukeland University Hospital, Bergen, Norway. 2 Institute of Physics and Technology, University of Bergen, Bergen, Norway Purpose/Objective: Online adaptive radiotherapy (ART) is dependent on daily re-contouring of targets. These are made on images with poorer quality compared to diagnostic imaging and under time-pressure from the patient laying on the treatment table. This study investigated delineation uncertainties in relation to intrafraction motion for cone beam CT (CBCT)- based ART in patients with locally advanced rectal cancer (LARC) receiving total neoadjuvant therapy. Material/Methods: Patients enrolling in the TNT-RECORD study (NCT05883800) receiving short-course ART were included in the analysis. In all adaptive fractions the oncologist modifies or approves the adapted clinical target volumes (CTV) before construction of the new plan (Fig.1). To evaluate delineation uncertainties, re-contouring of the CTV in