Bowel (Colonic) Polyps - Causes

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?

Large bowel showing a 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.

Hyperplastic polyps

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.

Adenomas

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

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.

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Author:
Dr Colin Tidy
Peer Reviewer:
Dr Adrian Bonsall
Document ID:
13486 (v4)
Last Checked:
02 November 2016
Next Review:
02 November 2019

Disclaimer: This article is for information only and should not be used for the diagnosis or treatment of medical conditions. Patient Platform Limited has used all reasonable care in compiling the information but make no warranty as to its accuracy. Consult a doctor or other health care professional for diagnosis and treatment of medical conditions. For details see our conditions.