S1011
Clinical – Paediatric tumours
ESTRO 2026
Between September 2023 and March 2025, a series of delineation workshops was held using two anonymised, previously treated cases: a pelvic ES undergoing radical radiotherapy and a chest-wall ES treated post-operatively. Requested structures were: GTVp_Pre, GTVp_Post, CTVp_Pre_4500 and CTVp_Post_5400 for both cases; plus CTVp_Post_6000 for pelvis and CTVp_Lung_1500 for chest-wall. Submitted contours were overlaid and visually inspected. A Frequency of Inclusion (FoI) mask was used to identify 80% of agreement (CV80%). Inter- observer variability was quantified using the Dice Similarity Coefficient (DSC) and Average Surface Distance (ASD). Results: For the pelvic case, the main discrepancy in delineation of GTVp_pre and GTVp_post (median DSC 0.68 and 0.63, maximum ASD 8.87 and 9.67 mm) was driven by the interpretation of the tumour extent on imaging. A high variability was highlighted for CTVp_Pre_4500, CTVp_Post_5400, and CTVp_Post_6000 volumes (figure 1), and confirmed by median DSC (0.75, 0.61 and 0.62) and maximum ASD values (17.6, 21.10 and 14.50 mm). Visual inspection revealed that variations were related to clinical decision-making when delineating, such as contour adjustments at anatomical barriers. Therefore, it was agreed to consider the compartments’ fascia as natural barriers in the absence of infiltration, while a 0.5 cm margin beyond the barrier should be applied if the ES is in direct contact.
Poster Discussion 2420 Challenges in delineation for Ewing Sarcoma of the chest wall and pelvis: results of a Euro-Ewing Consortium-QUARTET joint workshop Maria Chiara Lo Greco 1,2 , Volha Hertsyk 3 , Melissa Christiaens 4 , Line Claude 5 , Coreen Corning 3 , Emma D'Ippolito 6 , Raquel Dávila Fajardo 7 , Karin Dieckmann 8 , Jacob Engellau 9 , Jenny Gains 10 , Daniela Greto 11 , Yi-lan Lin 12 , Henriette Magelssen 13 , Henry C. Mandeville 10,14 , Valentine Martin 15 , Daiva Sendiuliene 16 , Beate Timmermann 12 , Andrada Turcas 17,18 , Lorna Zadravec Zaletel 19 , Sarah M. Kelly 1,2 , Akmal Safwat 20 1 QUARTET project, European Society for Paediatric Oncology, Brussels, Belgium. 2 Faculty of Medicine and Health Sciences, University of Ghent, Ghent, Belgium. 3 Medical Department, The European Organisation for Research and Treatment of Cancer (EORTC) Headquarters, Brussels, Belgium. 4 Gasthuisberg Campus, UZ Leuven, Leuven, Belgium. 5 Department of Radiation Oncology, Centre Léon Bérard, Lyon, France. 6 Department of Precision Medicine, University of Campania "L. Vanvitelli", Naples, Italy. 7 Department of Radiation Oncology, University Medical Centre Utrecht, Utrecht, Netherlands. 8 Department of Radiation Oncology, Medical University Vienna, Vienna, Austria. 9 Department of Oncology, Skane University Hospital, Lund, Sweden. 10 Department of Oncology, University College London Hospitals NHS Foundation Trust, London, United Kingdom. 11 Radiation Oncology Unit, Oncology Department, Azienda Ospedaliero Universitaria Careggi, Firenze, Italy. 12 University Hospital Essen, West German Proton Therapy Centre, Essen, Germany. 13 Department of Oncology, Oslo University Hospital, Oslo, Norway. 14 Children and Young People's Unit, Royal Marsden Hospital and Institute of Cancer Research, Sutton, United Kingdom. 15 Radiation Oncology Department, Gustave Roussy Cancer Center, Villejuif, France. 16 Department of External Beam Radiotherapy, National Cancer Institute, Vilnius, Lithuania. 17 Institute of Advanced Studies in Science and Technology, Babe ș -Bolyai University, Cluj-Napoca, Romania. 18 Radiotherapy Department, Oncology Institute “Prof. Dr. Ion Chiricuta”, Cluj-Napoca, Romania. 19 Radiotherapy Department, Institute of Oncology, Ljubljana, Slovenia. 20 Danish Center for Particle Therapy, Aarhus University Hospital, Aarhus, Denmark Purpose/Objective: Despite existing radiotherapy contouring guidelines for Ewing’s sarcoma (ES), important gaps remain for specific anatomical sites. The Euro-Ewing Consortium radiotherapy committee collaborated with QUARTET to identify ambiguities and address them through consensus statements. Material/Methods:
For the chest-wall case, variability in GTVp_Pre and GTVp_Post (median DSC 0.63 and 0.60, maximum ASD 7.75 and 5.55 mm) related to changes between pre- and post-operative anatomy. These ambiguities, such as the inclusion of surfaces initially in contact with the tumour, were reflected in CTVp_Pre_4500 and CTVp_Post_5400 (median DSC 0.77 and 0.76, maximum ASD 5.92 and 4.96 mm) delineation (figure 2). CTV_Lung_1500 showed the highest concordance
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