Epithelioid sarcoma is a unique malignant mesenchymal tumor with epithelial differentiation. It typically occurs in the distal extremities of young adults as a slow-growing nodule within the dermis, subcutis or the deep soft tissues. Two forms are recognized. The conventional or distal form is present as nodules composed of spindle-or-polygonal epithelioid cells with desmoplastic stroma of spindle cells in a storiform pattern surrounding the tumor. Central necrosis is very common. Hemorrhage, focal calcification, metaplastic ossification and myxoid change may also be seen. The proximal form arises in the deep part of the pelvis, perineum, genital tract and proximal extremities. It is characterized by large epithelioid carcinoma-like or rhabdoid cells, and has a more aggressive clinical course than distally located tumors. Moreover, it has a possible link with malignant rhabdoid tumor. The proximal form also shows greater pleomorphism and prominent nucleoli. In both forms, there is central tumor necrosis present in 70 to 80% of cases. Mitotic figure ranges from 1 to 50 per 10 high-power fields with a median of 9 mitotic figures. As a malignant tumor of mesenchymal origin, epithelioid sarcomas are positive for vimentin. But more importantly, they also express epithelial cell markers. Nearly all of epithelioid sarcomas are positive for epithelial membrane antigen and cytokeratin AE1 AE3. CD34 is positive 60 to 70% of cases. More than 80% of cases showed a loss of INI1 expression by immunohistochemistry. Tumor size and location are prognostic indicators. Epithelioid sarcoma tends to recur locally and metastasize via lymphatic system to regional lymph nodes, lung and other organs. The prognosis is poor with a median survival of 31 months. Main differential diagnosis should include malignant rhabdoid tumor, granuloma annulare, rheumatoid nodule, fibromatosis and angiosarcoma.
Reference / Suggested Readings
L. Chbani et al. Am J Clin Pathol 2009;131:222-227; 2009.
J. M. Orrock et al. Am J Dermatopathol. 2009; 31:152-156; 2009.
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