ESTRO 2025 - Abstract Book

S1649

Clinical – äediatric tumours

ESTRO 2025

3747

Poster Discussion Where Does Oligometastatic Pelvic Ewing’s Sarcoma Fit on the Spectrum of Disease? Alper Kahvecioglu 1 , Melis Gultekin 1 , Nur Nimet Saliha Akdag 1 , Mehmet Ayvaz 2 , Ali Varan 3 , Ferah Yildiz 1 1 Radiation Oncology, Hacettepe University Faculty of Medicine, Ankara, Turkey. 2 Orthopaedics and Traumatology, Hacettepe University Faculty of Medicine, Ankara, Turkey. 3 Pediatric Oncology, Hacettepe University Faculty of Medicine, Ankara, Turkey Purpose/Objective: This study evaluates the oncological outcomes of pediatric patients with pelvic Ewing’s sarcoma, a primary tumor site associated with a poorer prognosis compared to other anatomical locations. Specifically, the study investigates survival outcomes and the influence of metastatic burden in patients treated with radiotherapy (RT) as part of a multidisciplinary management strategy. Material/Methods: A retrospective cohort analysis was conducted involving 21 pediatric patients diagnosed with pelvic Ewing’s sarcoma and treated with RT between 2003 and 2024 at a tertiary cancer center. Oligometastatic disease was defined as the presence of ≤5 metastases. Overall survival (OS) was calculated from the date of initial diagnosis, while progression free survival (PFS) was calculated from the initiation of treatment. Results: Patient, tumor, and treatment characteristics are detailed in Table 1. Of the 21 patients, 5 (24%) presented with localized disease, 5 (24%) had oligometastatic disease, and 11 (52%) exhibited widespread metastatic disease. The most common metastatic sites were bone (43%), followed by lungs (40%). The median follow-up duration from the initial diagnosis was 36 months (range: 6–222 months). Eighty percent of patients with oligometastatic disease received metastasis-directed RT alongside local treatment of the primary tumor, which included surgery and/or RT. The 3-year OS rates were 100% for patients with localized and oligometastatic disease, compared to 11% for those with widespread metastatic disease ( p<0.001 ). Similarly, the 3-year PFS rates were 80%, 60%, and 0% for localized, oligometastatic, and widespread metastatic disease, respectively ( p<0.01 ). While 20% numerical difference in 3-year PFS rates observed between patients with localized disease and those with oligometastatic disease, it was not statistically significant (p=0.725). Univariate analysis results (Table 2) were followed by multivariate analysis, which identified widespread metastatic disease as the sole independent negative prognostic factor for OS (HR: 6.2, 95% CI: 1.5–2.5, p=0.01 ) and PFS (HR: 9.6, 95% CI: 2.1–4.2, p=0.03 ).