Cancer is when cells in the body change and grow out of control. To help you understand what happens when you have cancer, let's look at how your body works normally. Your body is made up of tiny building blocks called cells. Normal cells grow when your body needs them and die when your body does not need them any longer.
Cancer is made up of abnormal cells that grow even though your body doesn't need them. In most cancers, the abnormal cells grow to form a lump or mass called a tumor. If cancer cells are in the body long enough, they can grow into (invade) nearby areas. They can even spread to other parts of the body (metastasis).
The anus is the last part of the digestive or gastrointestinal tract. It’s at the end of the rectum. It's where stool comes out of the body. It consists of a muscular ring (sphincter) that opens during a bowel movement to allow stool (feces) to pass through. It’s also made of flat cells that form the lining of the inside of the anus. Most anal cancers start in these flat lining cells, which are also called squamous cells. The lower part of the anus, where it meets the skin, is called the anal margin.
Anal cancer is cancer that starts in the cells of the anus. Different types of tumors can form in the anus. Some of these tumors are not cancer. These tumors can include:
Benign or noncancerous anal tumors. Polyps are small growths that may be flat or bumpy. Or they 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 (fibroepithelial polyps) are examples of noncancerous (benign) polyps. Other types of benign tumors include adnexal tumors, leiomyomas, and granular cell tumors. They also include hemangiomas, lipomas, and schwannomas. These are all rare.
Anal warts (condylomas). These are noncancerous growths that may 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 anal warts are more likely to get anal cancer.
Precancerous lesions. Many people who get the most common type of anal cancer (squamous cell carcinoma) first had some patches of irregular cells. Your healthcare provider may call these lesions anal intraepithelial neoplasia (AIN). Each year, a small percentage of people with AIN get invasive cancer.
Malignant anal tumors. These are cancerous tumors. The most common type is squamous cell carcinoma. This type of anal cancer starts in the skin cells around the anus (called the perianal skin) and most of the anal canal. Adenocarcinomas are another type of anal tumor. They account for a small number of cases of anal cancer. Other types of malignant anal tumors are very rare.
If you have questions about anal cancer, talk with your healthcare provider. They can help you understand more about this cancer.