Answer: Melanoma in situ

Histology: There is pagetoid spread of atypical cells along the epidermal junction and extending up into the epidermis. Occasionally, nests of these cells are seen. The cells are hyperchromatic, angulated and large. Mitoses are also seen.

Discussion: The findings are consistent with melanoma in situ. Anal melanoma is rare and is associated with poor prognosis. Other entities on the differential diagnosis include Bowen’s disease, Paget’s disease and junctional nevus. There is too much atypia in the cells, along with mitoses, for the lesion to be considered a nevus. Bowen’s disease or in situ squamous carcinoma usually involves the full thickness of the epidermis and would not show the single atypical cells scattered in the epidermis, as seen in this case. Perianal Paget’s disease is a rare entity, and occurs in two settings. One is a malignant neoplasm of the intraepidermal portion of sweat glands. The other is associated with an underlying adenocarcinoma of the rectum or perianal glands. The cells of Paget’s disease are mucin producing, and can be helpful in differentiating it from melanoma.