Malignant melanoma with osteoclast-like giant cells: an unusual host response: immunohistochemical and ultrastructural study of three cases and literature review.

Melanomas with unusual histologic features are very rarely reported in the literature and demonstrate the diversity of melanocytic expression. Three cases of malignant melanoma with osteoclast-like giant cells are reported. Two cases showed undifferentiated malignant cells without melanin pigment and one showed spindled cell morphology. Immunohistochemistry showed that the osteoclast- like giant cells expressed CD68, but not melanocytic markers (HMB45, Melan-A, and S100). Ultrastructural analysis further supports that these cells are reactive histiocytes rather than transformed malignant cells. This suggests they represent an unusual host response, similar to those rarely observed in other neoplasms. Awareness of this entity is important to avoid misdiagnosis of melanoma as a histiocytic tumor. Since only few cases have been reported, greater recognition and documentation may help to evaluate the prognosis of such cases with unusual morphology.