"Histiocytic markers" in melanoma

Background: Tumor cells of malignant melanoma, the "great imitator," may mo rphologically mimic almost any cell, including histiocytes. Immunohistochem ical stains for histiocytes are often used to distinguish histiocytic lesio ns that resemble melanomas, but we have noted and others have reported that these markers may be immunoreactive in melanomas. Methods: We evaluated 43 primary and metastatic melanomas with traditional markers for melanomas (S 100, HMB45, and NKI-C3) and common markers used for histiocytes (alpha-1-an titrypsin or AAT, CD68/KP1, HAM56, Mac387, and Muramidase). The extent (< 5 %, 5 to 30%, 30 to 60%, 60 to 90%, >90%) and intensity (1+ to 4+) of staini ng were recorded semi-quantitatively. Results: Melanoma immunoreactivity (> 5% of tumor cells) was as follows: S100, 100%; HMB45, 91%; NKI, 91%; AAT, 9 5%; CD68, 86%; HAM56, 26%; Mac387, 7%; and Muramidase, 30%. Among the histi ocytic markers, staining by AAT and CD68 was typically diffuse but weak. St aining by HAM56, Mac387, and Muramidase was usually focal. In contrast, the traditional melanoma markers showed diffuse and strong staining. Interpret ation of the histiocytic markers was complicated by scattered atypical hist iocytes and pigmented tumor cells. Conclusion: Melanomas are commonly immun oreactive for histiocytic markers. AAT and CD68 immunostains are diffusely positive almost as frequently as traditional melanoma markers, although wit h weaker intensity. HAM56, Mac387, and Muramidase are less commonly positiv e and exhibit focal staining. Therefore, depending on the context, histiocy tic markers may not be helpful in differentiating histiocytes and histiocyt ic tumors from melanomas.