Colon polyps come in two forms, namely hyperplastic and adenomatous. Hyperplastic polyps are benign and do not indicate any future problems with colon cancer, whereas adenomatous polyps are precancerous and, if left in place, can eventually develop into colon cancer. These are the types of polyps that are sought during a flexible sigmoidoscopy or colonoscopy. Early removal of the polyp results in a significant decrease in the chances that a patient will develop colon cancer. People with a prior colon polyp have as much as a 50% chance of developing more colon polyps within three years of their diagnosis.
The cause of most colon polyps is unknown. A small number of people with inherited polyp syndromes are at much higher risk than the average person of having polyps and developing colon cancer.
A colon polyp usually is removed if it is adenomatous and larger than 1 cm (0.39 in.). Smaller polyps also are removed, although it is unclear whether polyps smaller than 5 mm (0.2 in.) indicate a risk of developing cancer. Although many people will develop colon polyps in their lifetime, most polyps will not develop into cancer. It is estimated that during one year's time, 1 out of every 400 adenomatous polyps will become cancerous.