Pathologic and Molecular Diagnosis of Ewing Sarcoma: A Multicenter Analysis From the Latin American Cooperative Group Trial.

Ewing Sarcoma (ES) is a small, round, blue cell tumor (SRBCT) characterized by a chromosomal translocation between chromosomes 11 and 22 in ~85% of cases, alongside immunohistochemical (IHC) expression of the surface glycoprotein CD99. Despite advancements in molecular diagnostics, low-income countries continue to face challenges in tumor classification and identification of fusion partners. This study retrospectively analyzed pathology reports from 396 patients enrolled in the Latin American Cooperative Group (GALOP) trial, with data collected until December 2021. CD99 positivity or molecular confirmation of EWSR1 translocation were required for inclusion. IHC marker selection varied across pathology units, reflecting differences in national guidelines. FLI1 was assessed in 45.5% of cases, VIM in 40.4%, and NKX2-2 in 14.9%. The most common complementary markers included desmin (60.1%), myogenin (47.5%), LCA/CD45 (51.5%), and synaptophysin (44.9%). EWSR1 translocation confirmation was performed in 74 patients (18.6%) using FISH and/or PCR. Molecular testing was more frequent in Argentina (73%), while Brazil, Chile, and Uruguay reserved it for diagnostically uncertain cases. Ki-67 was assessed in 70 cases, with most showing a high proliferation index (>30%). These findings underscore the need for continued collaboration to standardize diagnostic approaches across Latin America, aiming to improve treatment outcomes for ES patients.