Basal cell carcinoma of the anus: clinical and pathological distinction from cloacogenic carcinoma.

A 54-year-old man developed basal cell carcinoma of the anal verge. This extremely rare tumor behaves rather innocently, whereas cloacogenic carcinoma of the anus, which resembles it histologically, metastasizes early and often proves fatal. Clinical and histopathologic characteristics of the two anal neoplasms are tabulated to facilitate differential diagnosis, and thereby promote their very different therapies.