Immunohistochemical and histochemical tools in the diagnosis of amelanotic melanoma

The histologic diagnosis of (metastatic) oligomelanotic or amelanotic melanoma may be difficult. In most cases this diagnosis can be established with conventional light and electron microscopic examination, supplemented with staining for melanin on ultrathin sections, but in other cases it remains equivocal. Therefore, the melanoma-associated monoclonal antibody NKI/C-3, effective on paraffin sections, was tested with an indirect immunoperoxidase technique. All 19 metastatic melanomas, used as positive controls, were stained. Seventeen of 23 primary melanomas and 8 of 9 initially equivocal eventually unequivocal melanomas (Group I) were stained with a diffuse cytoplasmic and in some cases locally peripheral pattern. Only two large cell undifferentiated carcinomas of 58 histogenetically unrelated but differential diagnostically relevant tumors showed localized staining in few tumor cells. Furthermore, 10 of 20 histogenetically related tumors (neuroendocrine tumors and clear cell sarcomas) were positive. These tumors however, can easily be differentiated from melanomas by other means. Of 15 equivocal melanomas (Group II) 9 cases reacted with NKI/C-3, suggesting that it may be a useful marker for difficult metastatic tumors suspect for amelanotic melanoma. Although sensitivity of NKI/C-3 for metastatic melanomas is high, its specificity is not sufficient. It therefore can be applied most properly in a selected panel of different tumor-associated antibodies that are reactive in formaldehyde fixed, paraffin-embedded tissue.