ESTRO 2025 - Abstract Book

S4398

RTT - Treatment planning, OAR and target definitions

ESTRO 2025

Conclusion: The MRplanning GTV on average 3D T2-weighted MR does not capture all respiratory motion, as GTV-MRs are consistently smaller than ITV-CTs and consistent with GTV-CTs. Using margins from CINE MR and 4DCT to create MR-ITVs may be suboptimal due to characterizing motion with margin expansions and not 4DMR. Carefully applied ITV margins for MR-Linac treatments that use average scans will ensure target coverage, and results here indicate potential for online adjustments if treatment motion deviates from reference.

Keywords: ITV, MR-Linac, Kidney

4154

Digital Poster Upright radiotherapy for breast cancer: considering photon beam access and arm positioning Gordon D Sands 1 , Jemma Nunn 1 , Chung Tin Lo 2 , Simon Ingram 1 , Elizabeth Chabner 3 , Tracy Underwood 1,2 1 Translational Research, Leo Cancer Care, Crawley, United Kingdom. 2 Medical Physics and biomedical engineering, University College London, London, United Kingdom. 3 CEO, Masthead, ChabnerXRT, Scarsdale, USA Purpose/Objective: Upright radiotherapy may make conventional photon radiotherapy more cost-effective/accessible(1). If a seated patient is rotated through a fixed photon beam, radiotherapy equipment becomes smaller/lighter with only one direction of primary shielding. Previous work investigated the feasibility of upright positioning for breast radiotherapy, using bras as immobilisation accessories(2). With supine positioning, breast tissue falls laterally away from the patient's midline under gravity, whereas breast tissue lies more medially when upright. This raises questions over whether photon beams might clip the contralateral breast, with upright body positioning. Here surface scans were used to investigate. Material/Methods: After Research Ethics Committee approval, 15 healthy volunteers (aged 24-83, bra sizes B-H) were fitted with the customisable, indexable Chabner XRT Bra (Chabner XRT, LLC). Using custom-designed arms-up supports and a cutaway arms-back backrest, they trialled (i)arms-up and (ii)arms-back positions on a portable version of the Eve upright patient positioner (Leo Cancer Care). In each set-up, an Intel D415-3D camera was used with the itSees3d