Sarcoma of the right femur with heterogeneous morphology similar to myoepithelial carcinoma and amplification of the EWSR1 gene in a 14-year-old boy. Clinical observation and literature review

   Ewing sarcoma (ES) 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 ES, 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. In this article, 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.