CausesBy Mayo Clinic staff
The last part of your digestive tract is a long muscular tube called the large intestine. The colon makes up most of the large intestine. The rectum and anus make up the end of the large intestine. The colon's main function is to absorb water, salt and other minerals from colon contents. Your rectum stores waste until it's eliminated from your body as stool.
Why polyps form
The majority of polyps aren't cancerous (malignant). Yet like most cancers, polyps are the result of abnormal cell growth. Healthy cells grow and divide in an orderly way — a process that's controlled by two broad groups of genes. Mutations in any of these genes can cause cells to continue dividing even when new cells aren't needed. In the colon and rectum, this unregulated growth can cause polyps to form. Over a long period of time, some of these polyps may become malignant.
Polyps can develop anywhere in your large intestine. They can be small or large and flat (sessile) or mushroom shaped and attached to a stalk (pedunculated). In general, the larger a polyp, the greater the likelihood of cancer.
There are three main types of colon polyps:
- Adenomatous. About two-thirds of all polyps fall into this category. Although only a small percentage of these polyps actually become cancerous, nearly all malignant polyps are adenomatous.
- Hyperplastic. Most of the remaining polyps are hyperplastic. These polyps occur most often in your left (descending) colon and rectum. Usually less than a quarter of an inch (5 millimeters) in size, they're very rarely malignant.
- Inflammatory. These polyps may follow a bout of ulcerative colitis or Crohn's disease of the colon. Although the polyps themselves are not a significant threat, having ulcerative colitis or Crohn's disease of the colon increases your overall risk of colon cancer.
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