Metastases of malignant melanoma simulating soft tissue sarcoma. A clinico-pathological, light- and electron microscopic and immunohistochemical study of 21 cases.

Metastases of cutaneous malignant melanoma (MM) of ordinary type can resemble various types of soft tissue sarcoma light microscopically to a degree which has not been previously recognized. Twenty-one cases are described, in which the tumours were originally diagnosed as a soft tissue sarcoma. Seven tumours were predominantly of blue and spindle-cell, fascicular type, resembling malignant peripheral nerve sheath tumour and at times monophasic synovial sarcoma. Ten tumours which were of fascicular and predominantly storiform type, and included uni- and multi-nuleated pleomorphic cells resembled malignant fibrous histiocytoma. Due to the presence of multivacuolated lipoblast-like tumour cells, 2 of these 10 tumours resembled pleomorphic liposarcoma. One had a predominantly myxoid and hypocellular appearance and 5 additional tumours included such areas. The diagnoses were revised after ultrastructural examination with the demonstration of melanosomes in 13 of 16 studied cases and the immunohistochemical demonstration of positivity using anti-S-100 protein antibodies and the anti-melanoma antibody NKI/C3 in all cases. The anti-melanoma antibody HMB 45 gave a positivity in 9 of 21 cases. Light microscopically, sparse amounts of melanin were noted in 7 tumours using the Whartin-Starry technique. Eleven tumours occurred at sites close to major lymph node groups and in 9 of these cases, lymphoid tissue was associated with the tumours, suggesting that they represented lymph node metastases. Following a review of the patients' clinical histories and renewed clinical examination, primary cutaneous MM was demonstrated in 10 of 21 patients and in 1 case an MM in regression was detected. The origin of the 10 tumours without a detected primary is discussed, including the possibility of an overlooked primary, spontaneous regression of a primary and a de novo origin from lymph nodes and soft tissues.