S2136
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
References: [1] Hoyer M et al. Phase-II study on stereotactic radiotherapy of locally advanced pancreatic carcinoma. Radiother Oncol. 2005;76:48-53.[2] Mittauer KE et al. Online adaptive radiotherapy: Assessment of planning technique and its impact on longitudinal plan quality robustness in pancreatic cancer. Radiother Oncol. 2023;188:109869.[3] Grimbergen G et al. Treatment planning for MR-guided SBRT of pancreatic tumors on a 1.5 T MR-Linac: A global consensus protocol. Clin Transl Radiat Oncol. 2024;47:100797.[4] Paddick I. A simple scoring ratio to index the conformity of radiosurgical treatment plans. J Neurosurg. 2000;93:219–22. Keywords: MRI linac, Pancreatic cancer, Plan robustness Digital Poster 2257 Characterising intra and inter-fraction rectal motion for MR guided radiotherapy Benedict David Dobby 1 , Claire Nelder 1 , James Tallon 1 , Lisa McDaid 1 , Mairead Daly 2 , Frank Brewster 1 , Imogen Hemy 1 , Lucy Buckley 1 , Alice Greenwood-Wilson 1 , Claire Arthur 1 , Ananya Choudhury 3 , Cynthia Eccles 1,2 1 Radiotherapy, The Christie NHS Foundation Trust, Manchester, United Kingdom. 2 Division of Cancer Sciences, The University of Manchester, Manchester, United Kingdom. 3 Division of Cancer Sciences, The Christie NHS Foundation Trust, Manchester, United Kingdom Purpose/Objective: Variable, region-specific motion impacts the precision of MR-guided adaptive radiotherapy. This project quantified rectal motion along the whole rectum and its subregions to address this challenge. Material/Methods: Nine patients with locally advanced rectal cancer (8=T3N0M0, 1=T3N1M0) were recruited to an ethically approved imaging study (NCT02973828), between April 2021 and October 2023. Tumour locations were in the lower (n = 7), middle (n = 1), and upper (n = 1) rectum. All patients received radical chemoradiotherapy +/- (with curative intent) and underwent weekly MRI on a 1.5 T MR-Linac. At each session, axial T2-weighted images were acquired immediately post-setup (T0) and again after 10 minutes (T10) to simulate beam delivery.Images were rigidly registered to the planning scan (RTP) using pelvic bony anatomy. The length of the external rectal wall and upper, middle and lower rectal segments were contoured by 2 radiographers with the clinician RTP contour used as baseline. Segments were defined using anatomical landmarks published by Santiago et al(1) to assess regional variation. Consensus contours
were generated at each timepoint using simultaneous truth and performance level estimation (STAPLE)2. Rectal motion was quantified using the 95th percentile Hausdorff distance (HD95), mean distance to agreement (MDTA) and centre of mass shift (COM). Intra-fraction motion was calculated as the displacement in STAPLE rectal contours from T0 to T10. Inter-fraction motion was calculated as the displacement from the T0 rectal STAPLE contour compared to the clinicians RTP contour. Results: Median (range) HD95 and MDTA intra-fraction rectal motion was 4.9 mm (2.0 to 11.4) and 1.5 mm (0.6 to 4.7) respectively. HD95 and MDTA median (range) inter-fraction motion was 11.2 mm (4.8 to 20.2) and 4.3 mm (1.5 to 10.4). COM analysis showed the greatest motion in the upper rectum, with a median (range) Y axis shift of 0.3 mm (-0.60 to 2.10), and the least in the lower rectum, with a median (range) Y axis shift of 0 cm (-0.60 to 0.60). Motion occurred predominantly in the superior-inferior direction. No significant inter-patient variation was observed in intra-fraction motion (p = 0.48), whereas a significant difference was identified for inter-fraction motion (p < 0.001). Conclusion: Rectal motion varied across patients and regions, with the upper rectum demonstrating the largest displacement. While intra-fraction motion was stable, significant inter-fraction variation indicates that upper rectal tumours may benefit most from adaptive MRgRT. References: 1. Santiago I, Figueiredo N, Parés O, Matos C. MRI of rectal cancer — relevant anatomy and staging key points. Published online 2020.2. Warfield, S.K., Zou, K.H. and Wells, W.M., 2004. Simultaneous truth and performance level estimation (STAPLE): an algorithm for the validation of image segmentation. IEEE transactions on medical imaging, 23(7), pp.903-921. Keywords: Colorectal, Adaptive, Motion Generalising deep-learning models from open- source to multi-centre radiotherapy data in image- guided radiotherapy of young patients Ana-Cristina Ghica 1 , Mikaël Simard 1 , Shutong Yu 2 , Andrew Nisbet 1 , Jennifer Gains 3 , Pei Lim 3 , Yibao Zhang 2 , Catarina Veiga 1 1 Department of Medical Physics and Biomedical Engineering, University College London, London, United Kingdom. 2 Department of Radiation Oncology, Peking University Cancer Hospital, Beijing, China. 3 Department of Oncology, University College London Digital Poster 2299
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