What are they?
Bowel polyps have the appearance of surface elevations on the lining of the large intestine and range in size from a small cherry to a small mushroom (1-2mm to 5cm or more). The large majority of polyps do not become bowel cancer but essentially all bowel cancers arise from the polyps. Some polyps are quite flat on the lining of the bowel wall and others are attached to the bowel wall by a stalk, or pedicle. They occur as a result of repeated division of cells that should have been shed from the bowel lining as for normal renewal, but have stayed behind.
The two most common types of polyps are:
- adenomatous; and
Adenomatous polyps may develop into cancer but hyperplastic polyps do not. Sometimes polyps can be “mixed” (both hyperplastic and adenomatous) and these are also potentially malignant. The larger an adenoma becomes, the more likely it is to undergo malignant (cancerous) changes.
Multiple hyperplastic polyps have been shown to lead to a slightly increased incidence of colorectal cancer, as may sessile serrated polyps.
There are also other types of less common polyps, which have no relationship to cancer.
The majority of polyps do not cause any symptoms. The larger a polyp grows, the more likely it is to cause bleeding. Bowel polyps that are benign very rarely lead to bowel obstruction.
Most polyps can be removed with a colonoscope. A colonoscope is a long flexible instrument that can examine the entire length of the large bowel. If polyps are seen with the colonoscope they are almost always removed as it is not possible to tell which polyps have malignant potential or not. The larger a polyp, the more important it is that it be removed, as there is an increasing potential for malignancy as the polyp grows.
Polyps of less than 1cm diameter have a less than 1% chance of cancer, but this rises to 40% for polyps greater than 2cm.
If adenomas have been removed at colonoscopy, it is usually advisable for further colonoscopies to be performed. The frequency of these examinations will depend on the number and size of the polyps previously removed. For most routine situations, screening examinations at three yearly intervals is recommended.