Histologic findings of metastatic melanoma resemble the tumorigenic vertical growth phase of primary melanoma except that there is usually an absence of an inflammatory infiltrate and junctional activity. Although metastatic melanoma may manifest clinically as a pigmented nodule, similar to a dermatofibroma, it does not exhibit a spindle cell proliferation with collagen trapping typical of dermatofibromas. In addition, pigment in melanomas consists of melanin, not hemosiderin.
Primary nodular melanoma by definition contains only tumorigenic vertical growth phase, lacking an in situ component beyond the invasive vertical lesion. The dermal component is usually composed of atypical epithelioid and spindle-shaped melanocytes, which may be nested. The melanocytic cells are invariably immunoperoxidase positive for S-100 protein. A spindle cell proliferation with collagen trapping is not seen.
Dermatofibroma, sclerosing hemangioma type, is blue-black in color and clinically may be confused with a melanoma or blue nevus. Histologically, it resembles an “ordinary” dermatofibroma except that the epidermis is usually flattened, there is an increased number of vessels, and there are multiple histiocytes and multinucleated giant cells that contain abundant hemosiderin.
Blue nevi are characterized by pigmented spindle and dendritic melanocytes localized in the reticular dermis with alteration of dermal collagen. They contain melanin pigment, rather than hemosiderin, and lack collagen trapping by spindle cells.
Nodular lesions of Kaposi sarcoma, whether of the classic or AIDS-related type, are spindle cell proliferations that form vascular slitlike spaces with extravasated erythrocytes. Hemosiderin deposition is generally minimal, and histiocytes are absent. The spindle cells show positive human herpesvirus (HHV)-8 staining. Collagen trapping by spindle cells is not a feature.
