The anus is located at the end of the digestive tract below the rectum and is the opening to the outside of the body. The anal canal is a three to four centimeter long structure between the anal sphincter, a muscle that controls bowel continence, and the anal margin, the area of skin just outside of the digestive tract.
The two types of anal cancer are based on where the cancerous cells are located.
Anal Margin Cancer
Cancer of the anal margin is visible on the skin outside of the anus. This type of cancer is usually treated in the same way skin cancer is treated – by surgically excising the lesion, but not the anus itself.
Cancer of the anal margin should not be confused with melanoma of the anus.
Anal Canal Cancer
Cancer in the anal canal cannot be detected by sight. Anal canal cancer is usually detected with a digital rectal exam or a proctoscopy.