Ewing sarcoma is one of the most frequent primary bone tumors and has a well-studied diagnostically important genetic background. However, there are primary bone round-cell tumors with atypical morphology different from conventional Ewing sarcoma, tumors with rearrangement of the EWSR1 gene with partner genes not from the ETS gene family, tumors with unusual changes in the EWSR1 gene (amplification or deletion), which can cause significant diagnostic difficulties. We will describe a case of a primary bone tumor with an atypical morphology similar to myoepithelial carcinoma, where an amplification of the EWSR1 gene was detected. According to morphological, immunological, genetic and clinical signs, this tumor was classified as a sarcoma from the EWSR1::non-ETS group of round-cell sarcomas, namely a sarcoma with EWSR1::NFATC2 rearrangement, first formalized in the WHO classification of soft tissue tumors in 2020.

Ilya Sidorov is a pathologist at the Pathology Department of Dmitry Rogachev National Medical Research Center of Pediatric Hematology, Oncology and Immunology - the leading pediatric oncology medical center in Russia. He is an expert in bone and soft tissue tumors, and pediatric tumors. His research interests are undifferentiated round-cell sarcomas, author and co-author of 6 scientific articles.