Malignant melanomas simulating various types of soft tissue tumors.
BACKGROUND: Primary malignant melanomas, recurrences, and metastases thereof can present with a wide variety of clinicopathologic aspects. OBJECTIVE: The present series describes eight primary malignant melanomas and/or 10 local recurrences/metastases (from eight patients) misdiagnosed as various types of soft tissue tumors: two dermatofibrosarcoma protuberans, two atypical fibroxanthomas, two storiform-pleomorphic malignant fibrous histiocytomas, one myxofibrosarcoma ("myxoid malignant fibrous histiocytoma"), two malignant hemangiopericytomas, and nine malignant schwannomas. METHODS: In three cases correct diagnosis was established by clinicopathologic correlation during follow-up; all the others were only discovered during a retrospective work-up of all soft tissue tumors diagnosed at the Dermatohistopathological Laboratory of the Department of Dermatology, University of Innsbruck. RESULTS: Helpful clues derived from subtle intraepidermal, irregular spread of melanocytes, focal nested arrangement of tumor cells, sparse melanin deposition, neurotropism, and prominent peripheral lymphohistiocytic response, partially forming lymph follicles. Besides clinicopathologic correlation, histology of serial sections as well as immunohistochemistry (S100 protein more important than NK1/C3; HMB45 without benefit) and electron microscopy (melanosomes) proved helpful for definitive diagnosis. In contrast to the general assumption that spindle cell/desmoplastic malignant melanomas have an unequivocal bad prognosis, our series, as well as evidence from the literature, document a better prognosis than that of other types with comparable tumor thickness (70% vs 50% 5-year survival). Moreover, labeling with E9, an antimetallothionein marker, confirmed its usefulness for prognosis being strongly positive in primary lesions followed by rapid progression, but mostly negative in those without. CONCLUSION: These cases document that malignant melanoma may mimic various types of soft tissue tumors; correct interpretation is important as prognosis and therapeutic management differ considerably between these entities.