S2263
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
all combinations of superior/inferior (S/I), right/left (R/L), and anterior/posterior (A/P) anatomical directions originating from the structure’s geometric centre, resulting in the following sectors: RAS, RAI, RPS, RPI, LAS, LAI, LPS & LPI (Figure 1). For each structure and sector, the mean, median, and maximum voxel- wise radial distance differences relative to the geometric centre of the structure were calculated, with inward shifts (indicating no expansion beyond the original contour) set to zero.
Conclusion Sector-based analysis quantified direction-specific deformation. The GTV demonstrated smaller displacements than the whole bladder. The limited magnitude of GTV deformation suggests the feasibility of SBRT based on population-derived anisotropic margins. In the future, we will apply this methodology to additional patients with the aim of deriving personalised margins for bladder SBRT according to the bladder sector containing the tumour. References 1. Hafeez S et al. Bladder Tumor-Focused Adaptive Radiation Therapy: Clinical Outcomes of a Phase I Dose Escalation Study. Int J Radiat Oncol Biol Phys. 2025 Jan 1;121(1):165-175. 2. Huddart R et al, Dose-escalated Adaptive Radiotherapy for Bladder Cancer: Results of the Phase 2 RAIDER Randomised Controlled Trial. Eur Urol. 2025 Jan;87(1):60-70. 3. Mitchell A et al, Feasibility of tumour-focused adaptive radiotherapy for bladder cancer on the MR- linac. Clin Transl Radiat Oncol 2022, 35:27-32. Digital Poster 4022 Log-based quality assurance of Radixact Synchrony real-time adaptation deliveries Scott B Crowe 1,2 , Samuel C Peet 1 , Jemma Walsh 1,3 , Kate Stewart 1 , Tanya Kairn 1,2 1 Cancer Care Services, Royal Brisbane and Women's Hospital, Brisbane, Australia. 2 School of Chemistry and Physics, Queensland University of Technology, Brisbane, Australia. 3 School of Clinical Sciences, Queensland University of Technology, Brisbane, Australia
Results The mean ± standard deviation time from session to verification and post-treatment scans were 20 ± 4 minutes and 31 ± 8 minutes, respectively. At post- treatment scan, maximum bladder deformation occurred predominantly in the anterior/superior sectors, LAS: 18.1 ± 7.1 mm, RAS: 16.4 ± 8.7 mm. Posterior/inferior sectors showed smaller displacements (e.g., RPI: 6.6 ± 3.1 mm). The GTV exhibited smaller overall deformation than the bladder but followed the same anterior/superior expansion trends LAS: 10.7 ± 4.1 mm, RAS: 10.3 ± 3.8 mm with the posterior/inferior regions being relatively more stable (Figure 2A). Bladder showed higher median displacement than GTV in superior and posterior sectors. (Figure 2B). Post-treatment displacements exceeded those at the verification scans, reflecting continuing and monotonic bladder filling during the workflow.
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