Epithelioid variant of malignant peripheral nerve sheath tumor (malignant epithelioid schwannoma).

Twenty-six cases of malignant peripheral nerve sheath tumor with a predominant epithelioid pattern were studied to determine the range of its histologic patterns, immunophenotype, and biologic behavior. The tumor presented as an asymptomatic mass either in superficial (16 cases) or in deep soft tissue (10 cases) of the extremity. Characteristically, those in deep soft tissue were composed of vague nodules of varying cellularity made up of cords or strands of rounded epithelioid cells with prominent nucleoli. Those in superficial soft tissue were uninodular masses composed of tight clusters of cells showing cell-to-cell molding but possessing the same prominence of nuclei and mitotic activity as those in deep soft tissue. Several were associated with a preexisting benign nerve sheath tumor. A number of cases deviated from the above description, including cases that resembled a clear cell carcinoma, a malignant rhabdoid tumor, and a pleomorphic sarcoma. The majority of cases (80%) strongly expressed S-100 protein and neuron-specific enolase, but all lacked a melanoma-associated antigen (as defined by HMB-45) and cytokeratin. Stains for type IV collagen defined linear immunoreactivity around single cells and groups of cells. This pattern did not differ substantially from that of melanomas and therefore did not serve as a reliable discriminant. Follow-up information indicated a more favorable course for those in superficial soft tissue compared with those in deep sites. Two of 16 patients in the former group developed metastatic disease compared with three of 10 in the latter group. Tumors in superficial soft tissue may be eminently treatable and curable, depending on size.