ESTRO 2025 - Abstract Book

S1392

Clinical - Lung

ESTRO 2025

Keywords: NSCLC, oligometastasis, SBRT

3486

Digital Poster The importance of dose spill in lung SABR – a simulation study

Marcel Van Herk 1 , Azadeh Abravan 1 , Matthew Craddock 1 , Esther Troost 2 , Corinne Faivre-Finn 1 , Ursula Nestle 3 1 Radiotherapy Related Research Group, The University of Manchester, Manchester, United Kingdom. 2 OncoRay, National Center for Radiation Research in Oncology, Dresden, Germany. 3 Klinik für Strahlentherapie und Radiologische Onkologie, Kliniken Maria Hilf, Mönchengladbach, Germany Purpose/Objective: Stereotactic ablative radiotherapy (SABR) for early-stage lung cancer is characterised by high dose and high accuracy treatment delivery. In spite of pathology data showing microscopic disease (MD) at least 1 cm from the GTV 1 , small or no CTV margins are used. However, high rates of loco-regional control observed suggest that dose spill outside the PTV may be important. The purpose of this study is to simulate the impact of dose spill outside the PTV on local control and propose an alternative dose prescription that accounts for MD. Material/Methods: Our simulation framework calculates tumour control probability (TCP) based on an assumed tumour cell distribution and dose distribution, samples residual random and systematic errors after IGRT (S=0.2cm delineation variation, s=0.14cm intrafraction motion; 1cm respiration motion), calculates biologically effective dose (a/b=10Gy) and computes TCP from many potential dose-volume histograms (DVHs) 2 . Generic dose distributions of 5x12Gy were modelled on typical VMAT plans that consisted of Gaussian-blurred step function on the PTV (IGTV+8mm), on top of a 5x3.6Gy uniform background dose in the axial plane (Fig.1). MD was simulated from a layer of 50 3mm radius nodules 1 at fixed distance from the GTV in random directions. 5000 cases were simulated: random errors drawn per fraction and systematic errors per case. In addition, MD nodules are randomly enabled per case with 0.1 probability to enable on average 5 MD nodules in random directions 1 . Dose was accumulated and DVHs created of GTV plus enabled nodes.