Pigmented extramammary Paget disease of the abdomen: a potential mimicker of melanoma.

Extramammary Paget disease (EMPD) is a rare condition that usually presents in areas that are rich in apocrine sweat glands such as the vulva, scrotum, and perianal areas. The majority of these tumors represent cutaneous extension from a visceral adenocarcinoma, whereas a smaller proportion arise in the cutaneous apocrine glands themselves. Women in their sixth to eighth decades are most commonly affected. It is exceedingly rare for EMPD to present on the face, chest, or abdomen, and even more unusual for it to present as a pigmented lesion. We report the case of a 63-year-old woman with an underlying colon cancer who presented with a pigmented lesion in the midline of the abdomen above the umbilicus. Immunohistochemical stains demonstrated the lesion to be CK7+/CK20- as well as negative for melanocytic markers (S100, MiTF, Melan-A, HMB-45). Further, the immunophenotype of the EMPD differed from the patient's underlying colon adenocarcinoma (CK20+/CK7-), arguing against an ectopic focus of her established disease. Making the distinction between pigmented EMPD and melanoma is a potential diagnostic pitfall because of the histologic similarities. Extramammary Paget disease should be considered in the morphologic differential diagnosis of melanoma and, if necessary, supporting studies should performed to aid in this distinction.