ESTRO 2025 - Abstract Book

S3347

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

ESTRO 2025

effect. Effective beam-on time (including spot irradiation, spot switching, and burst switching times) and static delivery time were also compared between the two systems.

Results: With the same nominal plan, the IBA ProteusONE demonstrated a mean CTV D99% degradation of 1.83%, while the IBA ProteusPlus exhibited a greater target coverage degradation of 3.61%. For CTV D1%, the IBA ProteusONE showed an increase of 3.01%, compared to 5.11% for the IBA ProteusPlus. The IBA ProteusONE exhibited a smaller decrease in HI (-0.05) compared to the IBA ProteusPlus (-0.116). Similarly, CI decreased by 0.02 for the IBA ProteusONE and by 0.014 for the IBA ProteusPlus. The effective beam-on time was significantly longer for the IBA ProteusONE (45.55 seconds) compared to the IBA ProteusPlus (11.66 seconds). Static delivery times were also extended for the IBA ProteusONE (70.23 s) in comparison to the IBA ProteusPlus (41.91 s).

Conclusion: The interplay effect varies between the two proton systems equipped with different accelerators in lung cancer SBRT. The IBA ProteusONE offers better target dose “robustness” but at the cost of longer effective beam on time and delivery time compared to the IBA ProteusPlus system.

Keywords: IMPT, Lung SBRT

4279

Poster Discussion Motion mitigation in 2-fraction prostate SBRT using MRI-guided MLC-tracking Prescilla Uijtewaal 1 , Pim Borman 1 , Peter Woodhead 1,2 , Hans de Boer 1 , Jochem van der Voort van Zyp 1 , Bas Raaymakers 1 , Martin Fast 1 1 Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands. 2 Elekta, AB, Stockholm, Sweden Purpose/Objective: Intrafraction motion introduces uncertainty in prostate SBRT, with mainly rectal and bladder filling affecting the dose delivery to the prostate and organs at risk (OARs), especially for large fraction doses used in ultra hypofractionation (Fx=1-2). The 1.5T MR-linac is equipped with automatic gating and baseline shift plan adaptation capabilities based on a comprehensive motion manager (CMM) that tracks 3D target motion using cine-MRI. Recently CMM was upgraded with a new T2w cine-MRI sequence tailored for improved prostate visibility [1], but current motion management strategies still decrease delivery efficiency. This proof-of-concept phantom study