Cancer of the anus—the inch-long muscular tube where the rectum opens into the body surface—is less common than other bowel cancers, accounting for only about 1 to 2 percent of all cancers of the large bowel and its outlet. Fewer than 10,000 cases occur annually in the United States.
Anal cancer is highly treatable and often curable, especially in the early stages. The first sign of the cancer is often bleeding, which may be mistaken for hemorrhoids or benign anal disease. Anal cancers are often discovered by a pathologist in a specimen from a surgical operation done for apparently benign disease.
Types The vast majority—80 to 90 percent—of all primary cancers of the anus develop in the skin and are called squamous cell (epidermoid) carcinomas. These often first appear as an ulcer . An important subset is made up of tumors such as transitional cell, basaloid or cloacogenic cancer that can arise in the anal lining from embryonic tissue (cloacogenic) or the anal lining of transitional cells (basaloid tumors). There is also a squamous cell tumor within the anal lining (intraepithelial) called Bowen's disease that is a premalignant in situ squamous cell lesion of the skin around the anus. The third variety is mucoepidermoid anal cancer.
