What are the causes of bowel polyps?
The lining of the bowel constantly renews itself throughout our lives. New cells are made in the lining of the bowel and these cells eventually die to be replaced by more new cells. Each of these cells contains genes which control how the cell works.
If the gene doesn't work properly then some cells may grow more quickly. This may eventually cause a polyp to develop. Some faulty genes which cause a tendency for the lining of the bowel to develop polyps or bowel cancer may be inherited from our parents.
What are the types of bowel polyp?
Most bowel (colonic) polyps develop in the part of the colon called the descending colon, which leads down to the rectum. However, they can occur anywhere in the colon or rectum. Bowel polyps can be divided into three types: hyperplastic polyps, adenomas and polyposis syndromes.
These are common. They are small - usually less than half a centimetre in diameter. They are generally regarded as harmless and only very rarely develop into a cancerous (malignant) growth.
These too are common. Most are small (usually less than a centimetre in diameter) but they can be bigger. There is a small risk that an adenoma may, in time, become cancerous. If one does turn cancerous, the change usually takes place after a number of years. Most bowel cancers (colorectal cancers) develop from a polyp that has been present for 5-15 years. In other words, although most adenomas do not become cancerous, the vast majority of cancers in the bowel initially develop from an adenoma that has been present for a number of years.
It is difficult to be certain of the exact risk of developing cancer from an adenoma. One study suggested that for a single 1 cm adenoma, the risk of it turning cancerous was about 1 in 12 after 10 years, and about a 1 in 4 after 20 years. However, the risk seems to vary depending on factors such as the size of the adenoma and the exact type of adenoma. There are different types of adenoma and some types are more likely than others to develop into bowel cancer.
Polyposis syndromes are a group of inherited conditions. They include:
- Familial adenomatous polyposis (FAP).
- Hereditary nonpolyposis colorectal cancer (HNPCC)/Lynch's syndrome.
- Gardner's syndrome.
- Turcot's syndrome.
- Peutz-Jeghers syndrome.
- Cowden's disease.
- Familial juvenile polyposis.
- Hyperplastic polyposis.
These are rare syndromes. They cause polyps in young people and often cause multiple bowel polyps that have a high chance of developing into bowel cancer.
Further reading and references
Colonoscopic polypectomy and endoscopic mucosal resection: A practical guide; British Society of Gastroenterology (2008)
Guidelines for colorectal cancer screening and surveillance in moderate and high risk groups; British Society of Gastroenterology (May 2010 update from 2002)
Colonoscopic surveillance for prevention of colorectal cancer in people with ulcerative colitis, Crohn's disease or adenomas; NICE Clinical Guideline (March 2011)
Combined endoscopic and laparoscopic removal of colonic polyps; NICE Interventional Procedure Guidance, September 2014
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