ESTRO 2025 - Abstract Book

S417

Clinical - Biomarkers

ESTRO 2025

2498

Digital Poster Can pre-radiotherapy systemic immune-inflammation index predict prognosis in thymic epithelial tumors? Alper Kahvecioglu, Mustafa Cengiz, Mervenur Bay, Pervin Hurmuz Radiation Oncology, Hacettepe University Faculty of Medicine, Ankara, Turkey Purpose/Objective: The Systemic immune-inflammation index (SII) is a well-established prognostic biomarker in various solid tumors. However, the prognostic role of SII in thymic epithelial tumors (TETs) receiving adjuvant radiotherapy (RT) has not yet been explored. This study aims to assess the prognostic role of pre-RT SII in TET patients undergoing adjuvant RT. Material/Methods: A retrospective analysis was conducted on 37 patients treated with adjuvant RT for TETs between 2004 and 2023. Target volumes were defined based on the pre-operative tumor bed. The SII was calculated using the formula: [(Neutrophil x Platelet) / Lymphocyte]. All statistical analyses were conducted using the Statistical Package for the Social Sciences (SPSS), version 23.0 (IBM, Armonk, NY, USA). Results: Based on the AJCC/UICC 8th edition staging system, 73% were classified as stage group I, 16% as stage group II, and 11% as stage group IIIA. The median adjuvant RT dose was 54 Gy (range 50-70 Gy) delivered over 25-35 fractions, with a median follow-up time of 90 months (range 12-230 months). The 5-year overall survival (OS) and disease-free survival (DFS) rates were 97% and 91%, respectively. The ROC analysis determined an optimal SII cut-off value of 916 for predicting recurrence (AUC: 0.74, spesificity: 67%, sensitivity: 71%) (Figure 1). Patients with an SII ≥916 had significantly lower 5-year OS (92% vs. 100%, p=0.02) and DFS (73% vs. 100%, p=0.01) compared to those with a SII below 916 (Figure 2). Additionally, early stage disease and presence of myastenia gravis at diagnosis were associated with better DFS (p=0.01 and p=0.04).