A friend of mine had intestinal polyps and had to undergo surgery to remove a cancerous one. What causes polyps, and why are some cancerous? Are they preventable?

Don

Polyps are masses of tissue that protrude from the inner wall of the colon. Although they're very common among older people, it is not clear what causes polyps in most cases.

It is known that people with a family history of polyps or colon cancer face an increased risk of developing polyps. However, more than 95 percent of people with polyps do not have any family history. Other possible risk factors include a diet low in fiber, folate or calcium; a diet high in fat or alcohol; cigarette smoking; and obesity.

Not all polyps become cancerous. However, almost all cases of colon cancer develop in polyps -- usually in a particular type of polyp called adenomatous. Doctors still aren't clear why some polyps turn into cancer while others do not. We do know that large polyps (greater than 1cm) and those with a certain microscopic appearance (known as villous) are more likely to become cancerous than others. It is also known that the process is relatively slow. Many studies have shown that it takes at least four to five years -- and usually longer -- for cancer to develop in a polyp.

Currently, there's no proven way to prevent polyp formation. Although some studies claim that regular aspirin use or increased dietary intake of folate, fiber or calcium can reduce the risk of polyp formation, at this time no recommendations exist to support routine supplementation with these products.

So instead, we are directing our efforts at finding -- and removing -- polyps before they become cancerous. This early detection strategy is accomplished using a test called a colonoscopy. Colonoscopy involves placing a flexible tube (the colonoscope), with video capability, into the rectum. The scope is slowly advanced through the colon while the endoscopist looks for polyps. This exam is done with IV sedation to make the patient as comfortable as possible. Multiple studies have demonstrated that colonoscopy with removal of polyps can reduce the chance of developing colon cancer.

National guidelines recommend that people over age 50 or those with a family history of polyps or colon cancer should undergo colonoscopies. The findings at the initial colonoscopy dictate when the follow-up exam will need to be done. If no polyps are found, it is reasonable to wait five years before repeating the colonoscopy. Other groups may also benefit from regular screening, so you should be sure to discuss this issue with your doctor. Colon cancer is a preventable disease.