ESTRO 2025 - Abstract Book

S4051

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

ESTRO 2025

Conclusion: AC has limited effect on the position of intra-abdominal organs and did not reduce organ-to-organ distance. Baseline anatomy was the primary determinant of unfavourable OAR positioning. This was particularly evident with the bowel and liver, whose proximity to nominal TV remained unchanged even with the application of AC. Our data showed liver and spleen movement to be most influenced by AC in the superior/inferior direction, however, our results illustrate spleen motion in response to AC is unpredictable. In splenic radiotherapy this could impact accurate delivery of intended dose. At our centre, we utilise rapid reconstruction 4DCT to establish baseline anatomy and motion prior to applying AC. Future work includes the impact of AC on target volume and organs at risk for specific tumour sites, with a focus on their relative dosimetric relationship.

Keywords: Radiotherapy, OAR, organ motion

References: 1 Daly, M., McWilliam, A., Radhakrishna, G., Choudhury, A., & Eccles, C. L. (2022). Radiotherapy respiratory motion management in hepatobiliary and pancreatic malignancies: a systematic review of patient factors influencing effectiveness of motion reduction with abdominal compression. Acta Oncologica, 61(7), 833–841.

548

Digital Poster A feasibility study of Lung SABR in patients treated on a Halcyon machine with Surface-Guided Radiotherapy. Hannah Nayee, Antony Harris St Lukes Cancer Centre, Royal Surrey NHS Foundation Trust, Redhill, United Kingdom Purpose/Objective: The St Lukes Cancer Centre at Redhill was undergoing a year long machine replacement project, decommissioning 2 Varian Truebeams and installing 1 Varian Halcyon and Ethos machine with HyperSight imaging capabilities and AlignRT InBore surface-guided radiotherapy (SGRT) systems. Due to limited capacity during the replacement project, our SABR Lung patients were treated at the main site in Guildford which was approximately 28 miles away. When our replacement service was complete, we were able to reintroduce our Lung SABR service to Redhill and be the first in the UK to treat these patients with this new technology. Due to this, we were unable to gain advice from other UK departments on implementing this service. There are few Halcyon SABR lung research studies with SGRT (Kiser et al 2023). Material/Methods: A multi-disciplinary working party was created to overcome potential issues we would encounter in our workflow with the new machine. Due to the Halcyon having an automated field treatment function, we created test plans to ensure safety was a priority when taking interim scans for patient motion management. The test plans included a range of combinations of multiple isocenter plans, collimator angles and gantry angles to achieve a plan type which would be suitable to image these patients safely for their interim scans. The Halcyon machine does not have 4D CBCT capabilities, so other options such as a thorax slow mode was selected for these patients to track tumour motion, in addition to abdomen-compression devices for lower lobe patients to limit tumours from moving with breathing motion in this area. Results: It was found that the technique of treating patients with multiple isocenters caused a pause in between the treatment arcs, allowing for a safety measure to ensure the radiographers were able to take an interim scan