Soft tissue high grade myoepithelial carcinoma with round cell morphology: report of a newly described entity with EWSR1 gene rearrangement.

The case of soft tissue malignant myoepithelioma is presented including clinicopathological, immunohistochemical and cytogenetic findings. A 36-year-old Saudi male patient suffered from large mass involving right scapula and right shoulder joint measuring 14x13x11 mm. Core biopsy revealed sheets and lobules of poorly differentiated small malignant cells with marked atypia and frequent mitosis. Initially, immunohistochemistry was reactive for vimentin, pan-cytokeratin, EMA and CD99. The case was negative for desmin, SMA, CD34, S-100 protein and GFAP. FISH analysis exhibited negativity for SS18 (18q11.2) gene rearrangement and positivity for EWSR1 (22q12) gene rearrangement and a diagnosis of Ewing/PNET was considered. Clinical behavior and therapeutic response did not match the diagnosis with re-evaluation. Wedge biopsy demonstrated aggregates of epithelioid cells besides calponin and P63 positivity. Final diagnosis of malignant myoepithelioma with EWSR1 gene rearrangement was issued; a new entity with aggressive course. Myoepithelial carcinoma of soft tissue exhibits a wide spectrum of cytomorphology with overlapping phenotype similar to other soft tissue sarcoma like synovial sarcoma, mesenchymal chondrosarcoma, epithelioid sarcoma as well as Ewing/PNET. Moreover, a new finding of EWSR1 gene rearrangement is recognized in malignant myoepithelioma with different fusion partners. Hence, myoepithelial carcinoma should be kept in mind in diagnosis of soft tissue tumors even with unusual phenotype and gene rearrangement.