Challenges in delineation for Ewing Sarcoma of the chest wall and pelvis: Results of a QUARTET – Euro-Ewing Consortium joint workshop

Introduction: Despite existing radiotherapy contouring guidelines for Ewing's Sarcoma (ES), important gaps remain for certain anatomical sites. The Euro-Ewing Consortium radiotherapy committee collaborated with QUARTET to identify and address ambiguities through consensus statements. Material and methods: Between September 2023 and March 2025, a series of delineation workshops were held using two previously treated cases: a pelvic ES treated with radical radiotherapy and a chest wall ES treated post-operatively. The requested structures included GTVp_Pre, GTVp_Post, CTVp_Pre_4500 and CTVp_Post_5400 for both cases; as well as CTVp_Post_6000 for pelvic case and CTVp_Lung_1500 for chest wall case. Visual and quantitative analyses were performed. Results: For the pelvic case, substantial variability was observed in the CTVp_Pre_4500, CTVp_Post_5400, and CTVp_Post_6000 volumes. As differences were primarily related to adjustments at anatomical barriers, participants agreed to consider compartmental fascia as natural barriers in the absence of infiltration, and to apply a 0.5-cm margin beyond the barrier when the ES is in direct contact. For the chest wall case, variability in GTVp_Pre and GTVp_Post were related to changes between pre- and post-operative anatomy and reflected in CTVp_Pre_4500 and CTVp_Post_5400 delineation. It was therefore agreed that non-infiltrated surfaces in contact with the tumour before surgery should be included in the GTVp_Pre. Standard margins should be applied to generate CTVp_Pre_4500, which should then be edited to avoid the skin, lungs, and heart while ensuring inclusion of the surgical bed. Conclusions: The delineation workshops highlighted significant inter-clinician variability in target definition for ES. The resulting consensus statements will be used to supplement existing resources and support clinicians.