Although some people equate anal cancer with cancer of the colon, the two diseases are distinct and considered to be unrelated. The anus comprises the very end of the large intestine. It includes the anal canal (3-4 cm, approximately 1.5 inches), which is connected to the rectum, where some fecal waste is stored, and the anal opening, which allows fecal material to be excreted from the body. The colon comprises the final six-foot section of the large intestine up to the anal canal; the rectum makes up the final 8-10 inches of the colon. Colon cancer, also broadly known as colorectal cancer, was the second most common cancer (more than 224,000 cases) and the third most fatal form of cancer (more than 56,000 deaths) in the U.S. as of 1999. In contrast, approximately 500 deaths were attributed to anal cancer in the same year. Although this figure for anal cancer is relatively low, cases of the disease are concentrated-and incidence is high-in populations that are most at risk (e.g., MSM).

Risk factors for colon cancer include Crohn's disease (chronic inflammation of the intestines), chronic ulcerative colitis (ulcers in the large intestine linings), a history of breast cancer in women, increasing age, and family history of colorectal cancer. Scientists believe that dietary factors play a role in the development of colon cancer. These may include higher fat levels in the diet, low fiber intake, and the eating of grilled, smoked, or fried foods. The contrasting risk factors for anal cancer, such as infection with HPV and receptive anal intercourse, are outlined in the main article.

Despite their differences, one common risk factor for both anal and colon cancer-i.e., smoking-may play a larger role than previously imagined. Ann Chao, PhD, MPH, the lead author of a large prospective study (469,000 women and 312,000 men) conducted between 1982 and 1996, has claimed that as many as 12% of colorectal cancer-related deaths could be attributable to smoking. Though such extensive data are lacking in the case of anal cancer, a correlation between smoking and ASIL rates, as with CSIL rates, has been noted.

Special thanks to Dr. Joel Palefsky for his assistance in preparing this article.

Nicholas Cheonis is Associate Editor of BETA.

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