What Is Anal Cancer?

Anal cancer starts in the anus or anal canal. The anus is the opening at the end of your large intestine (the lower part of your digestive system). The anus leads from the intestine to the outside of the body and allows your stool (solid waste) to pass out of your body when you have a bowel movement. The anal canal is a 1½-inch segment of intestine just inside your anus. 

Anal cancer begins when the cells of the anus or anal canal grow and divide without stopping. These cells create lumps (masses) called tumors. Cancerous tumor cells can spread to other parts of the body if they are not treated and eliminated. 

Anal cancer is relatively rare. Approximately 5,070 new cases of anal cancer will be diagnosed in the United States in 2008, according to the American Cancer Society (ACS). Women are at slightly higher risk of anal cancer than men and the average of diagnosis is in the early 60s, according to the ACS.

The Digestive System and the Anus

The anus is at the very end of the large intestine—a winding organ typically 5 to 6 feet long. The large intestine is the lower part of your digestive system. The digestive system processes food to collect energy and gets rid of solid waste from the body. The digestive system is also known as the gastrointestinal tract or GI tract.

After you chew and swallow food, the food goes through the esophagus to the stomach. The food is partly broken down in the stomach and then goes to the small intestine, or small bowel, which is narrower than the large intestine. The small intestine continues to break down the food and absorbs most of its nutrients. The small intestine joins the colon in the lower right abdomen.

The colon is also known as the large intestine or large bowel. It is a muscular tube that takes up the first 4 or 5 feet of the large intestine. The colon absorbs water and minerals from broken down food and stores waste.

The waste that is left after this process is stool, which goes into the rectum. The rectum is the last 6 to 10 inches of the large intestine. The waste passes out of the body from the rectum through the anus.

The anus is made partly out of the outer skin layers of the body and partly out of the intestine. Two ringlike muscles (sphincter muscles) open and close the anal opening to let stool pass out of the body. The anal canal, the part of the anus between the rectum and the anal opening, is about 1½ inches long.

Because the anus is located partly inside the body and partly outside of the body, it is made up of different types of cells. Each type of cell can become cancerous.

How Anal Cancer Starts

Any cancer that starts in the anus or the skin around the anus is called anal cancer. This cancer is formed by abnormal cells that grow and divide without stopping. The cancer cells replace normal cells and form a lump (tumor). As a tumor gets bigger, it can grow into nearby tissues and organs. Cancer cells can spread to other parts of the body through the blood or lymph vessels. Most anal cancers start in the cells that line the inside of the anus. Tumor cells grow there, invade the layers of the anus, and spread to lymph nodes and other tissues.

Scientists believe that anal cancers often start as anal dysplasia (also known as intraepithelial neoplasia or AIN), or abnormal changes in the cells of the anus without any invasion into the surrounding tissue. In the most serious type of anal dysplasia, carcinoma in situ, the cells look like cancer cells but are found only in the lining of the anus and have not spread from the anus. Over time, anal dysplasia eventually become invasive and becomes able to break away to other parts of the body (metastasize).

If anal cancer metastasizes outside the anus, cancer cells often travel to nearby lymph nodes (tiny bean-shaped organs throughout the body that help fight infections), nerves, or blood vessels. If the cancer reaches these lymph nodes, it can also spread to other lymph nodes or other organs, such as the lungs or liver.

When cancer spreads from its original place to another part of the body, the new tumor has the same kind of abnormal cells and the same name as the original tumor. For example, if anal cancer spreads to the lungs, the cancer cells in the lungs are anal cancer cells. The disease is metastatic anal cancer, not lung cancer, and it is treated as anal cancer.

Types of Anal Cancer

Anal tumors can be benign (noncancerous) or malignant (cancerous). Certain kinds of growths, such as polyps or warts, sometimes start in or around the anus and can become cancerous. The most common kinds of anal cancer are:

• Squamous cell carcinoma - Squamous cell carcinomas account for about 80 percent of all anal cancers. These tumors start in the squamous cells that line most of the anal canal.
  • Carcinoma in situ - In its earliest stage, squamous cell carcinoma is known as carcinoma in situ (CIS) or Bowen disease. In carcinoma in situ, the cancerous cells are found only in the lining of the anus and have not spread into any other tissues.

  • Invasive squamous cell carcinoma - The cells have spread beyond the lining of the anal canal.

• Cloacogenic carcinoma - This squamous cell cancer develops in the area between the outer part of the anus and the lower part of the rectum.

• Adenocarcinoma - A small number of anal cancers are known as adenocarcinomas. Adenocarcinomas can develop in glands that make mucus, or in a type of sweat gland (apocrine gland) located in the skin surrounding the anus. These cancers usually start in the rectum and are treated in the same way as rectal carcinomas.

  • Paget’s disease - Paget’s disease is a type of apocrine gland carcinoma that spreads through the surface layer of the skin. Paget’s disease can affect skin anywhere in the body, but usually affects skin of the perianal area (near the anus), vulva (area around the vagina), and breast. This condition is different from Paget’s disease of the bone, a different disease also named after Dr. James Paget.

• Basal cell carcinoma - Basal cell carcinoma is a type of skin cancer that can start in the perianal skin (near the anus). These tumors are much more common in areas of skin that are exposed to the sun, such as the face and hands, and account for only a small percentage of anal cancers.

• Malignant melanoma - This cancer starts in cells in the skin or anal lining that produce the brown pigment called melanin. About 1 percent to 2 percent of anal cancers are melanomas, which are much more common on parts of the body that are exposed to sun. If melanomas are found at an early stage, before they have invaded deeply into the skin or spread to lymph nodes, the outlook for long-term survival is excellent.

Key Statistics

According to the American Cancer Society, anal cancer affects more women than men. Of the 5,070 new cases expected in 2008, 3,050 will occur in women and 2,020 in men. Women are more likely to have cancers in the inner part of the anus (anal canal), while anal tumors in men tend to develop on the outside of the anus.

Anal cancer is more common in older people; patients with anal cancer have an average age in the early 60s. The vast majority (about 85 percent) of people with anal cancer are Caucasian.

Treatment for anal cancer is very effective and most cases of anal cancer are cured.

This content has been reviewed and approved by Myo Thant, MD.