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Malignant melanoma of soft parts: an ultrastructural study of four cases.

Malignant melanomas of soft parts from 4 patients were studied by light microscopy, immunocytochemistry for S-100 protein, and electron microscopy. Each patient presented with a deep soft tissue mass in an extremity. Histologically, the tumors were composed of epithelioid and spindle cells, and in one, neoplastic giant cells were present. The tumors did not stain for melanin but were all positive for S-100 protein. Ultrastructurally, premelanosomes were identified in every tumor and in a cell line established from one tumor. Schwann cell features were present in one of the tumors. Although the clinical presentation of malignant melanoma of soft parts is similar to that of epithelioid sarcoma and synovial sarcoma, the combined light microscopic, immunocytochemical, and ultrastructural features should serve to distinguish it from other soft tissue sarcomas.