S2126
Physics - Inter-fraction motion management and daily adaptive radiotherapy
ESTRO 2026
and penumbra widths were measured. Candidate anisotropic margins were tested by eroding the PTV to form evaluation CTVs (CTV_eval). Monte-Carlo simulations modeled systematic ( Σ ) shifts and Gaussian random ( σ′ ) setup blurring. Two acceptability criteria were applied: (i) CTV_eval Dmin0.03cc ≥ 90 % Rx in ≥ 90 % of simulated scenarios, and (ii) population tumour-control-probability (TCP) loss < 1 %. Results: VMAT plans exhibited intrinsic 90 % isodose-to-PTV clearances of 3–5 mm laterally and anteriorly and 1–2 mm superior-inferiorly. The axial-plane penumbra (11– 16 mm in left, right, and anterior; 5–6 mm posterior) was substantially broader than van Herk’s 3.2 mm assumption. With an anisotropic [0, 0, 2] mm (LR/AP/SI) CTV-to-PTV margin, ≥ 90 % of patients maintained Dmin0.03cc ≥ 90 % Rx provided Σ lay within an ellipsoid of [2.0, 1.5, 1.8] mm and σ′ ≤ [1.5, 2.0, 1.5] mm. Under the TCP < 1 % criterion, the safe Σ - ellipsoid for high-risk disease was [2.5, 1.9, 2.2] mm, whereas low-intermediate-risk cases were even less sensitive to geometric uncertainty. Conclusion: For image-guided coplanar VMAT prostate radiotherapy, an anisotropic [0, 0, 2] mm CTV-to-PTV margin is sufficient to maintain both minimal-dose and TCP-based coverage criteria. The classic van Herk formula overestimates margins under modern VMAT conditions because its original assumptions of a tight prescription isodose and uniform 3.2 mm penumbra no longer hold. References: 1. van Herk, M., et al., The probability of correct target dosage: dose-population histograms for deriving treatment margins in radiotherapy. Int J Radiat Oncol Biol Phys, 2000. 47(4): p. 1121-35.2. van Herk, M., P. Remeijer, and J.V. Lebesque, Inclusion of geometric uncertainties in treatment plan evaluation. Int J Radiat
Table 2: Comparison of Bowel Dose Threshold Percentages Between Scheduled and Adapted PlansThe entire oART workflow was completed within 10 to 20 minutes, depending on the accuracy of the auto-Contouring. Conclusion: AI-driven CBCT-guided oART is feasible for bladder cancer radiotherapy treatment including a SIB. oART significantly improve plan quality. Our results support the integration of oART into routine clinical practice. References: Azzarouali, S., et al. (2023). Online adaptive radiotherapy for bladder cancer using a simultaneous integrated boost and fiducial markers.Archambault, Y., et al. (2020). Making on-line adaptive radiotherapy possible using artificial intelligence and machine learning for efficient daily re-planning. Med Phys Int J, 8.̊Astr om, L. M., et al. (2022). Online adaptive radiotherapy of urinary bladder cancer with full re- optimization to the anatomy of the day: Initial experience and dosimetric benefits. Radiotherapy and Oncology.Burridge, N., et al. (2006). Online adaptive radiotherapy of the bladder: Small bowel irradiated- volume reduction. International Journal of Radiation Oncology* Biology* Physics. Keywords: Bladder cancer, Reoptimization, CBCT Digital Poster Highlight 1436 Revisiting van Herk’s margin formula in co-planar VMAT prostate radiotherapy Quan Chen, Ali Ammar, Nathan Y Yu, Libing Zhu, Yi Rong, Carlos E Vargas Radiation Oncology, Mayo Clinic, Phoenix, USA Purpose/Objective: The classic van Herk margin formula was derived under two assumptions: (i) the planned isodose line closely follows the PTV surface, and (ii) the dose penumbra has a uniform σ ≈ 3.2 mm. This study investigated whether that formula overestimates clinical target volume (CTV) to planning target volume (PTV) expansions for contemporary coplanar volumetric-modulated arc therapy (VMAT) prostate treatments. Material/Methods: Fifty consecutive prostate stereotactic body radiotherapy (SBRT) VMAT plans (two coplanar arcs; clinical margins 3 mm except 2 mm posterior) were analyzed. Direction-specific 90 % isodose-to-PTV gaps
Oncol Biol Phys, 2002. 52(5): p. 1407-22. Keywords: PTV margin, prostate, VMAT
Poster Discussion 1501 Adaptive Fractionation for Tumors Overlapping with OARs Yoel Pérez Haas, Lena Kretzschmar, Bertrand Pouymayou, Stephanie Tanadini-Lang, Jan Unkelbach Radiation Oncology, University Hospital, Zurich, Switzerland Purpose/Objective: In abdominal tumors, the planning target volume (PTV) often overlaps with organs-at-risk (OARs) such as the stomach, bowel, or duodenum. In these cases, the prescribed dose frequently exceeds OAR constraints, leading to local underdosage of the PTV. The extent of
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