Most cancers of the colon begin as noncancerous (benign) polyps, which are growths on the inner surface of the tubelike large intestine. There are several different kinds of polyps, some of which may become cancerous growths and some of which rarely do. Doctors use a procedure known as colonoscopy (examination of the inside of the colon with a lighted tube) to check for polyps because removing a benign polyp prevents it from ever changing into a cancerous growth. After removing a benign polyp, doctors often repeat the colonoscopy periodically (surveillance colonoscopy) to ensure that no additional polyps have appeared. Several professional societies have written guidelines that advise doctors how often to repeat colonoscopy, depending on the chances that a particular kind of polyp is likely to recur and become cancerous. Because colonoscopy is expensive and requires a lot of physician time, it is important to tailor the frequency of surveillance colonoscopy to the likelihood that colon polyps will recur.