ESTRO 2025 - Abstract Book

S3269

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

ESTRO 2025

Conclusion: Breathing motion and intra-fraction drift require substantial PTV margins of 6 mm. Applying drift correction and gating effectively reduces the required PTV margin. Even relatively infrequent drift corrections result in substantial margin reductions. These findings enable optimization of motion-compensation parameters, balancing efficiency and accuracy in MRIgRT.

Keywords: tracking, gating, liver SBRT, PTV margins

References: [1] van de Lindt T, Sonke JJ, Nowee M, Jansen E, van Pelt V, van der Heide U, et al. A Self-Sorting Coronal 4D-MRI Method for Daily Image Guidance of Liver Lesions on an MR-LINAC. Int J Radiat Oncol Biol Phys 2018;102:875–84. [2] van de Lindt TN, Fast MF, van Kranen SR, Nowee ME, Jansen EM, van der Heide UA, et al. MRI-guided mid-position liver radiotherapy: Validation of image processing and registration steps. Radiother Oncol 2019.

2610

Digital Poster Intrafractional motion of SABR lung tumours due to a gravity effect Lars Merring-Mikkelsen, Hella Maria Brøgger Sand, Mads Høyrup Brincker Department of Medical Physics, Aalborg University Hospital, Aalborg, Denmark

Purpose/Objective: Intrafractional motion during lung Stereotactic Ablative Radiotherapy (SABR) is influenced by multiple factors that pose a significant challenge to treatment precision. While respiratory motion is well documented [1], the impact of gravity on tumour motion has not yet been quantitatively reported in scientific literature. However, clinical cases with large tumour shifts during treatment, have been observed at our clinic [2]. This study aims to quantify gravity induced tumour shifts during SABR lung treatments and identify potential contributing factors. Material/Methods: For 48 SABR lung patients treated with 67.5 Gy in 3 fractions in free-breathing, pre- and post-treatment CBCTs were acquired during the first two fractions. Tumour position on both CBCTs were determined through CBCT-to-CT