CEA reactivity in amelanotic malignant melanoma of the anal canal.

Amelanotic malignant melanoma can present a diagnostic problem in histopathology, especially when it presents in an extracutaneous location. In such cases electron microscopy and/or immunohistochemistry are invaluable for diagnosis. A 63-year-old women with rectal bleeding was found to have an amelanotic malignant melanoma of the anal canal, with compatible clinical and gross pathology and light and electron microscopic findings. Tumor cells showed positive staining with antibodies to vimentin, S 100 protein, and HMB-45 (melanoma-specific antibody), but also with polyclonal antibody to carcinoembryonic antigen (CEA). Tumor cells failed to stain with monoclonal antibody to CEA. The nature and significance of CEA reactivity in malignant melanoma are discussed.