Epidermotropic metastatic epithelioid sarcoma: a potential diagnostic pitfall.

Epithelioid sarcoma (ES) represents an aggressive soft tissue tumor with varied morphologic and histopathologic presentations that typically elicits a broad differential diagnosis, including granuloma annulare, necrobiotic granuloma, fibrous histiocytoma, synovial sarcoma, amelanotic melanoma and poorly differentiated primary cutaneous and metastatic adenocarcinoma. ES is characterized microscopically by a nodular arrangement of abundant, deeply eosinophilic, polygonal tumor cells with frequent central necrosis and hemorrhage, rare mitotic figures and minimal pleomorphism. At the periphery, tumor cells are spindle shaped and may exhibit frequent local infiltration along tendons, fascial planes and neurovascular bundles. Immunohistochemistry typically reveals expression of both epithelial and mesenchymal antigens, such as cytokeratin and vimentin, respectively. The absence of a connection between tumor cells and the overlying epidermis, with or without an in situ carcinoma component, typically rules out a primary cutaneous squamous cell carcinoma. We report a case of stage IV proximal-type ES that mimicked molluscum contagiosum clinically and was histopathologically reminiscent of invasive squamous cell carcinoma because of attachment and colonization of the overlying epidermis. The case represents an unusual pathologic presentation of ES and highlights potential pitfalls in establishing the diagnosis.