Anal Cancer

What is the anus?
The anus is an organ that lies at the end of the digestive tract below the rectum. It consists of two sections: the anal canal and the anus (or anal verge). The anal canal is a 3-4 cm long structure that lies between the anal sphincter (one of the muscles controlling bowel movements) just below the rectum and the anal verge which represents the transition point between the digestive tract and the skin on the outside of the body. Muscles within the anal canal and anus control the passage of stool from the rectum to outside the body.

What is anal cancer?
Normally, cells in the body will grow and divide to replace old or damaged cells in the body. This growth is highly regulated, and once enough cells are produced to replace the old ones, normal cells stop dividing. Tumors occur when there is an error in this regulation and cells continue to grow in an uncontrolled way. Tumors can either be benign or malignant. Although benign tumors may grow in an uncontrolled fashion sometimes, they do not spread beyond the part of the body where they started (metastasize) and do not invade into surrounding tissues. Malignant tumors, however, will grow in such a way that they invade and damage other tissues around them. They also may spread to other parts of the body, usually through the blood stream or through the lymphatic system where the lymph nodes are located. Over time, the cells within a malignant tumor become more abnormal and appear less like normal cells. This change in the appearance of cancer cells is called the tumor grade, and cancer cells are described as being well-differentiated, moderately-differentiated, poorly-differentiated, or undifferentiated. Well-differentiated cells are quite normal appearing and resemble the normal cells from which they originated. Undifferentiated cells are cells that have become so abnormal that often we cannot tell what types of cells they started from.
Anal cancer is a malignant tumor of either the anal canal or anal verge. In the United States, 80% of anal cancers are squamous cell cancers,, resembling the cells found in the anal canal., This is not true in other parts of the world, however. In Japan, 80% of anal cancers are adenocarcinomas, resembling the glandular cells seen in the rectum. Cancers of the anal verge may be referred to as “perianal skin cancers,” because they usually behave more like skin cancers than like anal cancers. They may respond more poorly to treatment than other forms of anal cancers. Perianal skin cancers represent about 25% of all anal cancers. Occasionally, other types of cancer, such as melanoma, Kaposi’s sarcoma, and lymphoma may develop in the anus. These other types of cancer will be discussed separately, and will not be addressed further in this review.
Anal cancers frequently begin as anal dysplasia. Anal dysplasia is made up of cells of the anus that have abnormal changes, but do not show evidence of invasion into the surrounding tissue. The most severe form of anal dysplasia is called carcinoma in situ. In the case of carcinoma in situ, cells have become cancerous, but have not begun to invade normal tissue yet. Over time, anal dysplasia changes to the point where cells become invasive and gain the ability to metastasize, or break way to other parts of the body. Anal dysplasia is sometimes referred to as anal intraepithelial neoplasia (AIN), or a “pre-cancer”. When anal cancer does spread, it most commonly spreads through direct invasion into the surrounding tissue or through the lymphatic system. Spread of anal cancer through the blood is less common, although it can occur.

What causes anal cancer and am I at risk?
Each year, there are approximately 4,000 cases of anal cancer in the United States. In general, the incidence of anal cancers has been increasing over the past 30-40 years. The vast majority (~85%) of cases are in Caucasians. The incidence of anal cancer increases with age: patients with anal cancer have an average (median) age of 62 years. Cancers of the anal canal are more common in women, while the incidence of cancers of the anal verge is roughly equal in both men and women.
Several factors have been associated with anal cancer. Most importantly, infection with the human papilloma virus (HPV) has been shown to be related to anal cancers and has been associated with several other cancers including cervical cancer and cancers of the head and neck. HPV can be transmitted from person to person through sexual contact, so individuals with a history of multiple sexual partners, anal receptive intercourse, and genital warts are at an increased risk for infection. Probably due to the association between HPV and anal cancer, women with history of cervical cancer are at increased risk of developing anal cancer. Another sexually transmitted virus, the human immunodeficiency virus (HIV), has been linked to anal cancers, and individuals infected with HIV are at increased risk for infection with HPV. The relationship between HIV and anal cancer will be discussed in more detail in the next section (entitled "How are anal cancer and HIV/AIDS related?")
Several other factors have been linked to anal cancer. Anal cancer has been associated with smoking. Patients who smoke are three times more likely to develop anal cancer as those that don't smoke. The risk of anal cancer increases with the number of cigarettes smoked per day and the number of years that a person has been smoking.
There may be an association between anal cancer and suppression of the immune system. The rate of anal cancer is higher in patients who are immunosuppressed after organ transplants, although this relationship is not clear.
Although there appears to be an increased rate of anal cancer in patients who have benign anal conditions such as anal fistulae, anal fissures, perianal abscesses, or hemorrhoids, it does not appear that these benign conditions are a cause of anal cancer. Alternatively, an undiagnosed anal cancer may actually be causing these conditions, and then is subsequently diagnosed when the benign condition is being treated.