ESTRO 2025 - Abstract Book

S3236

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

ESTRO 2025

There was a significant difference in the total GTVs between FM and DM: 3.9 cm³ in FM and 5.3 cm³ in DM for lung cancer (P < 0.001), and 31.5 cm³ in FM and 40.7 cm³ in DM for pancreatic cancer (P = 0.004).

Conclusion: The intra-fractional variations in DM were generally less than 5 mm, although they were larger than those in FM, particularly during the inspiratory phases, suggesting its potential as an alternative surrogate.

Keywords: Dynamic tumor tracking radiotherapy, Diaphragm

1823

Digital Poster Optimisation of CTV to PTV margins for a more personalised approach to MR-guided bladder radiotherapy Hannah Carter 1 , Pooja Gohil 1 , Joe Wood 1 , Marcel van Herk 2 , Ananya Choudhury 3 , Peter Hoskin 3 , Robert Chuter 1,2 1 Christie Medical Physics and Engineering, The Christie NHS Foundation Trust, Manchester, United Kingdom. 2 Faculty of Biology, Medicine and Health, The University of Manchester, Manchester, United Kingdom. 3 Clinical Oncology, The Christie NHS Foundation Trust, Manchester, United Kingdom Purpose/Objective: There is no consensus CTV to PTV margin for MR-guided radiotherapy for bladder cancer, therefore standard of care margins for non-adaptive treatments at this centre are used (1.5cm). It is hypothesised that this margin is larger than necessary, hence, by utilising the extensive imaging data available when performing MR-guided radiotherapy, this project derived an empirical population-based margin and established the feasibility of a personalised margin workflow. Material/Methods: Ten patients receiving radiotherapy of 55Gy in 20# to a whole bladder volume were retrospectively analysed using ten fractions per patient. Bidirectional local distance (BLD) statistics were calculated comparing the bladder contour from the pre-treat scan to the contour from the post-treat scan [1], quantifying intra-fraction motion. To investigate anisotropic margins, BLD data were binned by anatomical direction and considered separately. Inter-observer delineation uncertainty was calculated using contours delineated by a multidisciplinary team of 2 physicists and 3 radiographers. A population margin was calculated using the van Herk equation [2], incorporating systematic and random intra-fraction motion and inter-observer delineation uncertainty. To establish if bladder filling can be predicted for each patient based on initial fractions, the cumulative average of BLD values in each anatomical direction was investigated. The average BLD value after five fractions was combined with inter-observer variability to create a personalised margin. These margins were applied to patients pre treatment bladder contour and visually assessed on post-treatment images to estimate coverage. Results: The calculated population-based margin is shown in the table below. These margins are smaller in all directions compared to current standard of care margins of 1.5cm. Note that the margins depend on the workflow timing (Fig1). Direction Proposed margin (cm) Superior 0.9 Inferior 0.3 Anterior 0.7 Posterior 0.8 Right 0.6 Left 0.6