Cancer of this organ, which is also called the large intestine, is a common malignancy in the developed world. In the United States, for example, a person faces a 5 percent lifetime risk of developing colon cancer. Along with cancer of the rectum, colon cancer is the third most common cancer in both men and women in the United States. Colorectal cancer is the second leading cause of cancer deaths among men in this country, and the third among women. There will be 93,800 new cases of colon cancer and 47,700 deaths from the disease in the U.S. this year.

The colon is divided into four sections: the ascending colon, which passes upward from a pouch called the cecum along the right side of the abdomen; the transverse colon, which crosses the top of the abdomen; the descending colon, which runs down the left side; and the sigmoid colon, the last few inches of which are called the rectum.

About 95 percent of colon cancers first appear as adenomas, or polyps (adematous polyps), which are benign outgrowths from the lining or mucosal layer of the intestine.

A series of genetic mutations, some congenital and some acquired, interact with environmental factors to trigger the development of these polyps and their progression to cancer. The steps of this process are fairly well understood, and may one day be used in early detection of colon cancer and gene-based therapy.

The lining of the large intestine is continually exposed to the food we consume, and diet appears to play a major role in the development and prevention of colon cancer.

Removing polyps can prevent or cure the disease if the cancer has not spread further, so screening for them is the best way to prevent colon cancer. Screening is performed with fecal occult blood tests to detect hidden blood in the stool; sigmoidoscopy, in which a lighted tube is inserted into the rectum to view the lining of the lower end of the colon; and colonoscopy, a similar exam that allows the doctor to examine the entire colon.