Myoepithelial carcinoma of the uterine corpus with EWSR1::ZNF444 fusion: a case report.

Myoepithelial carcinoma (MECA) is a rare malignant tumor showing myoepithelial differentiation, most commonly found in the salivary glands and soft tissue, with only isolated reports in the female genital tract. We report the first documented case of uterine MECA harboring an EWSR1::ZNF444 fusion. A 55-year-old woman with a history of breast cancer presented with abnormal uterine bleeding. Imaging suggested a benign lesion, but hysteroscopic resection raised suspicion for uterine sarcoma. Hysterectomy revealed an infiltrative neoplasm composed of epithelioid and spindle cells with focal osseous differentiation. Immunohistochemistry showed focal positivity for αSMA, desmin, AE1/AE3, CAM5.2, and S100. Recurrent disease in lymph nodes and omentum showed a similar phenotype. FISH demonstrated EWSR1 rearrangement, and RNA sequencing identified an in-frame EWSR1::ZNF444 fusion, confirming the diagnosis of MECA. The tumor recurred twice over two years, reflecting aggressive behavior. While MECA typically arises in salivary glands and soft tissue, this case expands its anatomical spectrum to the uterus. This report underscores the importance of molecular diagnostics in classifying rare uterine mesenchymal tumors and highlights the diagnostic challenges posed by their broad morphological and immunophenotypic overlap with other entities.