Malignant melanoma with osteoid formation.

We report a case of malignant melanoma with osteoid formation arising in a patient who had had a diagnosis of neurofibromatosis. The presence of osteoid was confirmed histochemically by the picrosirius-polarization method and by transmission electron microscopy. Osteoid formation in malignant melanoma is rare, with only three cases reported prior to ours. Neoplasms arising from derivatives of the neural crest are known to show areas of metaplastic differentiation and heterologous elements. Among neural tumors, malignant nerve sheath neoplasms in particular are noted to display such heterologous elements as rhabdomyoblasts, cartilage, osteoid, and bone. Such heterologous elements are much less common in malignant melanoma. Our case report is an unusual example of a neural crest-derived neoplasm with heterologous elements.