Both flat and protruding polyps can develop into cancer, according to colon cancer expert Suzanne Dixon for About.com. The flat polyps, also called sessile polyps, rest on the mucus membrane inside the colon. Unlike the protruding, or pedunculated, polyps that look like mushrooms, flat polyps do not have a stalk.
Recent studies have found that flat polyps are possibly more common than researchers previously thought. Because of their shape, it is often more difficult to find these polyps during the screening process. There is no reason to assume, though, that a diagnosis with sessile polyps means the patient should dispense with further screening, notes Dixon.
While flat and pedunculated are the two shapes of polyps, the four types are inflammatory, adenomatous, hyperplastic and villous. Inflammatory polyps show up most frequently in people suffering from ulcerative colitis, Crohn's disease or another inflammatory bowel disease. These are benign and usually have no risk of turning into colon cancer, explains Dixon.
About seven of every 10 polyps in the typical colon are adenomatous, states Dixon. These can turn into colon cancer, but it usually takes years. Regular screening for colon cancer finds and eliminates these polyps before they become cancerous. Hyperplastic polyps have cells that increase rapidly in number, but they still present less of a risk of cancer than adenomatous polyps. Villous polyps are the most likely to turn into colon cancer, and many of them are flat, making them tougher to find and remove. Larger villous polyps sometimes need surgery for total removal.