S2262
Physics - Intra-fraction motion management and real-time adaptive radiotherapy
ESTRO 2026
parameters according to RAVENTA [1] for VMAT plans without, with apical and with parasternal US-probe sparing. Conclusion: The US-probe could successfully be spared from the primary beam without compromising STAR plan quality beyond minor, clinically acceptable DVH deviations. Additional obstacles, such as the presence of an S-ICD, may constitute a special challenge for planning. References: [1] Blanck O, Buergy D, Vens M, Eidinger L, Zaman A, Krug D, et al. Radiosurgery for ventricular tachycardia: preclinical and clinical evidence and study design for a German multi-center multi-platform feasibility trial (RAVENTA). Clinical Research in Cardiology 2020;109:1319-32. https://doi.org/10.1007/s00392- 020-01650-9. Keywords: STAR, ultrasound-based cardiorespiratory tracking Poster Discussion 4012 Modelling tumour deformation for MR-guided bladder SBRT Stefanos Diamantopoulos 1 , Adam Robert Mitchell 1 , Simeon Nill 1 , Uwe Oelfke 1 , Robert A Huddart 2 , Shaista Hafeez 2 1 The Joint Department of Physics, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom. 2 Urology Unit, The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London, United Kingdom Purpose/Objective Adaptive tumour focused bladder radiotherapy is challenged by changes occurring in the filling and position of the whole bladder [1-3]. Understanding these changes is an essential step towards assessing how bladder wall deformation influences gross tumour volume (GTV) size and position. This study quantifies both the whole bladder and GTV intra- fractional deformation using a 3D sector-based model. We aim to determine an anisotropic margin strategy for a potential bladder tumour focused adaptive SBRT approach. Material/Methods Six patients with unifocal bladder cancer treated on the MR-Linac within the PERMIT (NCT03727698) and MOMENTUM (NCT04075305) studies were selected for analysis. The Bladder and GTV were contoured, on the session, verification and post treatment scans. To assess the deformation of these structures, a sector- based deformation model was developed. The 3D contours were divided into eight octants according to
Figure 1: VMAT configurations for one PTV. A: 3D- reconstruction with apical placement of the US-probe. B: Configuration without US-probe. C/D: US-probe in apical/parasternal position. Results: Dose guidance parameters for OOI could be fulfilled in all cases without major protocol deviations, with the exception of the maximum doses for coronaries and aorta, which were caused by overlaps with the PTV and are clinically intended (Table 1). Additionally, one treatment plan with parasternal US-probe exceeded Bronchi V9Gy slightly with 5.9ccm. Treatment plans without and with apical US-probe had the same number of major protocol deviations (six for coronary and two for aorta D0.035ccm). Plans with parasternal US-probe exceeded coronary and aorta D0.035ccm in seven and two cases, respectively. Median D98%(PTV) deviated by less than 0.2Gy between the different configurations. US-probe and implanted cardioverter defibrillators (ICD) were successfully spared from the primary beam with D0.035ccm ≤ 0.5Gy in ten cases. For one patient with a subcutaneous-ICD (S-ICD) direct proximate to left ventricle and PTV, US-probe D0.035ccm was 0.1Gy (apical) and 1.5Gy (parasternal). S-ICD D0.035ccm was approximately 5Gy with and 0.7Gy without US-probe sparing.
Table 1: Sample median [minimum-maximum] of DVH
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