SEER Training Modules

Introduction to Colorectal Cancer

Although colorectal cancer is the third most common cancer—with an estimated 147,500 new cases expected to be diagnosed in 2003 (72,800 men and 74,700 women)—its incidence among Americans is decreasing. The mortality rate is also decreasing, which may reflect advances in detection and screening as well as the increasing use of combination therapies. Nevertheless, recurrence continues to be a serious problem.

Cancer can affect the colon or the rectum, the last 20-25 centimeters of the colon. Because cancer often affects both areas, it is frequently referred to as colorectal cancer. Anal cancer is an uncommon disease in which malignant cells are found in the anus. The anus is the opening at the end of the rectum (the end part of the large intestine) through which body waste passes. Cancer in the outer part of the anus is more likely to occur in men; cancer of the inner part of the rectum (anal canal) is more likely to occur in women.

Americans have about a one in 20 lifetime risk of developing colorectal cancer. It affects primarily those over 65, but risk starts increasing at age 40. Only about 3 percent of these malignancies occur in patients under 40. Incidence of colorectal cancer is nearly the same among men and women until age 50, when it becomes slightly higher among men.

The exact cause of colorectal cancer is unknown, however at least eight different genes involved can be traced to dietary fat, particularly animal fat. During fat metabolism, bacteria in the bowel form carcinogens (cancer-causing agents) that can irritate the intestinal lining. It is believed that polyps form in response to this irritation. These are often a precursor of cancer.

A high-fiber diet is thought to be somewhat protective because it helps accelerate the rate at which fats pass through the bowel and/or dilutes the concentration of fats, reducing the exposure of the large intestine to carcinogens. This theory is based on various epidemiological studies. However, clinical trials are underway that are designed to demonstrate whether there is any benefit, such as preventing polyps, from increasing the fiber content of the diet.