Cancer of the Anus

The anus is the body's opening at the lower end of the intestines. The anal canal is the tube that connects the lower part of the large intestine (rectum) to the anus and the outside of the body. As food is digested, it passes from the stomach to the small intestine. It then travels from the small intestine into the large intestine (colon). The colon absorbs water and liquid from the digested food. The waste matter that is left after going through the colon is known as feces or stool. Feces are stored in the rectum, the final 6 inches of the digestive system. From there, they pass out of the body through the anus as a bowel movement.
The anal canal is about an inch and a half long. Its inner lining (called the mucosa) is made up of several different kinds of cells. Learning a little about these cells is helpful in understanding the kinds of cancer that develop in various parts of the anal canal. Glands and ducts (tubes leading from the glands) are found under the mucosa. These glands make mucus, which acts as a lubricating fluid.
The anal canal goes from the rectum to the anal margin (where the canal meets the outside skin at the anus). About midway down the anal canal is the dentate line, which is where most of these anal glands empty into the anus.
Cells above the anal canal (in the rectum) are mainly shaped like tiny columns, while most of those in the upper anal canal just above the dentate line are shaped like cubes and are called transitional cells. This area is called the transitional zone. Below the dentate line are flat (squamous) cells. The anal margin (also called the anal verge) is lined by squamous cells that merge with the skin just outside the anus (called perianal skin). The perianal skin is also made up of squamous cells, but it also contains sweat glands and hair follicles; the lining of the anal canal does not. The place where the perianal skin (which has squamous cells plus hair follicles and sweat glands) meets the anal canal (which has squamous cells without hair follicles or sweat glands) is called the anal margin or anal verge. Cancers of the anal canal (above the anal verge) and cancers of the anal margin (below the anal verge) are treated very differently.
The anal canal is surrounded by a sphincter, which is a circular muscle that keeps feces from coming out until it is relaxed during a bowel movement.

Anal tumors

Many types of tumors can develop in the anus. Not all of these tumors are cancers -- some are benign (non-cancerous). There are also some growths that start off as benign but over time can develop into cancer. These are called pre-cancerous conditions. This section discusses all of these types of abnormal growths.