A colonoscopy visually examines the walls of the colon for cancer and polyps, growths that may become cancer. “All colon cancer starts as polyps,” says Zuri Murrell, M.D., director of the Colorectal Cancer Center at Cedars-Sinai. While other screening tests find cancer, a colonoscopy is both a screening and prevention test. If your physician finds polyps, she can remove them during the colonoscopy, preventing them from eventually becoming a cancer, he says. There are several steps involved in preparing for a colonoscopy.