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Causes
By Mayo Clinic staff
Your digestive tract stretches from your mouth to your anus. As food travels along this 30-foot (9-meter) passageway, nutrients are broken down into a form that can be absorbed by your body and used to build cells and produce energy.
The last part of your digestive tract is a long muscular tube called the large intestine. The colon is the upper 4 to 6 feet (1 to 2 meters) of the large intestine; the rectum makes up the lower 8 to 10 inches (20 to 25 centimeters). 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.
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). Small and mushroom-shaped polyps are much less likely to become malignant than flat or large ones are. 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 under 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 0.5 centimeters (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.
- Ahnen DJ, et al. Approach to the patient with colonic polyps. http://www.uptodate.com/home/index.html. Accessed June 15, 2009.
- Ahnen DJ, et al. Epidemiology and risk factors for colorectal cancer. http://www.uptodate.com/home/index.html. Accessed June 15, 2009.
- What I need to know about colon polyps. National Digestive Diseases Information Clearinghouse. http://digestive.niddk.nih.gov/ddiseases/pubs/colonpolyps_ez/. Accessed June 14, 2009.
- Torpy JM, et al. Colon polyps. Journal of the American Medical Association. 2008;300:1480.
- Fletcher RH. Screening for colorectal cancer: Strategies in patients at average risk. http://www.uptodate.com/home/index.html. Accessed June 14, 2009.
- Levin B, et al. Screening and surveillance for the early detection of colorectal cancer and adenomatous polyps, 2008: A joint guideline from the American Cancer Society, the U.S. Multi-Society Task Force on Colorectal Cancer, and the American College of Radiology. CA, A Cancer Journal for Clinicians. 2008;58:130.
- Colorectal cancer screening. National Cancer Institute. http://www.cancer.gov/cancertopics/factsheet/Detection/colorectal-screening. Accessed June 15, 2009.
- How is colorectal cancer found? American Cancer Society. http://www.cancer.org/docroot/CRI/CRI_2_1x.asp?rnav=criov&dt=10. Accessed June 15, 2009.
- Seitz U, et al. Endoscopic removal of large colonic polyps. http://www.uptodate.com/home/index.html. Accessed June 15, 2009.
- How is colorectal cancer treated? American Cancer Society. http://www.cancer.org/docroot/CRI/content/CRI_2_2_4X_How_Is_Colorectal_Cancer_Treated.asp?sitearea=. Accessed June 15, 2009.
- Can colorectal cancer be prevented? American Cancer Society. Accessed June 15, 2009.
- Picco MF (expert opinion). Mayo Clinic, Jacksonville, Fla. June 19, 2009.
- MYH-associated polyposis. Cancer Net, American Society of Clinical Oncology. http://www.cancer.net/patient/Cancer+Types/MYH-Associated+Polyposis. Accessed June 19, 2009.
