S1957
Physics - Dose prediction/calculation, optimisation and applications for photon and electron planning
ESTRO 2026
Conclusion: Dosimetric descriptions of BOS lesions are affected by the algorithm used. In the case of optic path meningiomas, tumours stabilised despite the apparent lower dose coverage. This suggests that the nominal prescribed dose could be lowered when Monte Carlo is used to avoid inadvertent increase in toxicities. This patient cohort will be monitored for pituitary related toxicities. A larger series of lesions treated in heterogeneous environments will allow for optimization of treatment and improvement of the risk:benefit profile of radiotherapy for these conditions. Keywords: base of skull lesions, prescription dose GTV dose robustness Across Respiratory Phases in Lung SBRT with GTV-based prescription: a subset of ongoing multicohort investigation Volha Hertsyk 1 , Angela Sofia Martins Ferreira 2 , Nicolaus Andratschke 3 , Enrico Clementel 1 , Younes Jourani 2 , Bryan Leurquin 1 , Nick Reynaert 2 1 Headquarters, EORTC, Brussels, Belgium. 2 Medical Physics Department, Institut Jules Bordet, Hôpital Universitaire de Bruxelles, Brussels, Belgium. 3 Department of Radiation Oncology, University Hospital Zürich, Zürich, Switzerland Purpose/Objective: Previous studies have demonstrated that GTV dose varies substantially under PTV prescription1. ICRU 91 recommended reporting GTV median dose in stereotactic body radiotherapy (SBRT) plans2,3 due its variability in lung particularly. Additionally, plan Digital Poster 3848 complexity may further amplify differences between planned and delivered doses particularly in cases with larger tumour motion amplitudes4. This sub-study evaluates the robustness of GTV-based prescription across respiratory phases in lung SBRT, assessing whether a density-override optimisation strategy maintains consistent GTV coverage throughout the breathing cycle. Material/Methods: Five lung SBRT cases with 4DCT datasets were retrospectively selected, prioritising lesions with the largest tumour motion amplitudes to simulate a worst- case scenario. Four CTs were used for dose calculation: the clinical free-breathing CT and 4DCT phases (maximum inhale, maximum exhale, mid- ventilation). Treatment planning (Monaco TPS, XVMC) employed single half-arc VMAT (6 FFF) with prescription dose 4x12Gy. All lesions were located peripherally in the right lung. The GTV encompassed the visible tumour, the PTV was generated from ITV (+0.5cm in all directions). Planning used a water-
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