Osteogenic melanoma. A rare variant of malignant melanoma.

Osteogenic melanoma is a rare variant of malignant melanoma; only eight cases have been reported. To characterize this unusual neoplasm further, we present four new cases. Two patients were men and two were women (average age, 56 years; range, 47-78 years). All tumors arose from acral lentiginous melanomas. Three were subungual finger lesions and one was on the sole of the foot. All four had been previously diagnosed as or were suspected to have been primary osseous lesions. The vertical growth components were high-grade, amelanotic sarcomatoid malignancies with abundant osteoid matrix. Two tumors also had chondroblastic differentiation. Cells with epithelioid features, including prominent eosinophilic nucleoli, were discernible in every tumor. Regional lymph node metastases in two cases retained osteocartilaginous differentiation, whereas metastatic cells in another case were purely epithelioid. Tumor cells in every case were immunoreactive for S-100 protein and vimentin, and non-reactive for cytokeratin. Two tumors also expressed HMB-45. Melanosomes were identified ultrastructurally in every tumor. Follow-up information was available on every patient. Three developed regional lymph node metastases and are currently alive and well after 14, 39, and 101 months. The fourth patient died of metastatic uterine carcinoma 20 months postoperatively. The differential diagnosis of osteogenic melanoma includes osteosarcoma as well as atypical fibro-osseous proliferations. Clinico-pathologic features that support a diagnosis of osteogenic melanoma include junctional activity, absence of primary bony involvement, regional nodal metastases, immunoreactivity for S-100 protein and/or HMB-45, lack of cytokeratin reactivity, and ultrastructural identification of melanosomes.