The most frequent type of anal cancer is squamous cell carcinoma, described in a specific chapter (see entry “ Squamous Cell Carcinoma, Anus”). Other types of anal carcinoma, i.e., adenocarcinoma, neuroendocrine carcinoma, Paget’s disease, and basal cell carcinoma, can also be observed.
Adenocarcinoma of the anal canal is a malignant epithelial tumor arising from glandular cells lining the upper anorectal part of the anal canal, anal transitional zone (ATZ) mucosa, the anal glands, or the epithelium lining of fistulous tracts.
Most adenocarcinomas found in the anal canal represent downward spread from a rectal adenocarcinoma or arise from colorectal-type mucosa lining the ATZ above the dentate line. Rare cases, described as extramucosal perianal adenocarcinomas, develop in pre-existing congenital or acquired anal sinuses or fistulae (in patients with Crohn’s disease). Some adenocarcinomas develop from anal glands or ducts that drain the anal...