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What is Anal Cancer

What Is the Anus?

The anus is the end of the large intestine, below the rectum, through which stool (solid waste) leaves the body. The anus is formed partly from the outer, skin layers of the body and partly from the intestine. Two ring-like muscles, called 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.

The skin around the outside of the anus is called the perianal area. Tumors in this area are skin tumors, not anal cancer.

What Anal Cancer Is

Anal cancer is a disease in which malignant (cancer) cells form in the tissues of the anus. Anal cancer is a rare cancer. Most people who get it are between ages 50 and 80. Slightly more women than men get anal cancer. But the rate is increasing in men, especially in those who have anal sex.

Types

Anal cancer is cancer that starts in the anus. Different types of tumors can form in the anus. Some of these tumors are not cancerous and some are cancerous.

Benign or Noncancerous Anal Tumors

Polyps are small growths that may be flat or bumpy or may look like mushrooms. They are not cancer. There are different types of polyps depending on their location and their cause. Inflammatory polyps, lymphoid polyps, and skin tags (also called fibroepithelial polyps) are examples of different kinds of noncancerous polyps. Other types of benign tumors include adnexal tumors, leiomyomas, granular cell tumors, hemangiomas, lipomas, and schwannomas. These are rare.

Anal Warts (Condylomas)

These are growths that usually occur just outside the anus and in the lower anal canal. They are caused by infection with a human papilloma virus (HPV). People who have had condylomas are more likely to develop anal cancer.

Precancerous Lesions

Many people who develop the most common type of anal cancer first had some patches of irregular cells. Your doctor may call these lesions anal intraepithelial neoplasia (AIN). Each year, a small percentage of people with AIN develop invasive cancer.

Malignant Anal Tumors

These are cancerous tumors. There are a number of different types, most of which are described in the list below. Other types of malignant anal tumors are very rare.

Squamous Cell Carcinoma

This is the most common type of tumor. This type of anal cancer begins in the cells that line the anal margin and most of the anal canal. 

Cloacogenic Carcinomas (also called transitional cell carcinomas)

These make up most of the remainder of anal cancers. This cancer type affects the cloaca, the bottom part of the large intestine that stores the stool.  

Adenocarcinomas

These account for a small number of cases of anal cancer, and affect the anal glands. Paget's Disease (not the same as the Paget's bone disease or Paget's disease of the breast) is a type of adenocarcinoma.

Basal Cell Carcinoma and Melanoma

These are types of skin cancers that can rarely cause anal cancer.

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