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Most colorectal cancers develop from a polyp. A polyp is a mass of cells that forms and grows in the tissues that cover the mucous membranes of some body cavities.
There are different types of polyp. Adenomatous polyps, or adenomas, are most commonly associated with the development of colorectal cancer (70–85% of cases). Serrated polyps have also been described as cancer precursors in up to 15–30% of cases.
Although most polyps do not progress to a tumour (less than 5–10% are estimated to lead to cancer), over 90% of colorectal cancers have probably gone through this stage at some time in their development.
Adenomas can be categorised as low or high risk depending on the probability of progression to cancer. This risk of progression is determined by the number of polyps (greater risk when three or more are present), their size (greater when they are over 1 cm) and a hairy appearance or high-grade dysplasia (anomaly in its development) (both are microscopic characteristics, assessed by pathologists after polyp resection).
Given the difficulty in distinguishing between different types of polyp from a macroscopic view, that is, based on the images from a colonoscopy, all polyps should be removed and undergo histological analysis (biopsy). Rectal polyps measuring just a few millimetres are the only possible exception to this recommendation as they have no potential to generate into a tumour.
The progression from a normal mucous membrane to the appearance of cancer, passing through the polyp stage, is estimated to take over 10 years. It could be much quicker under certain circumstances, such as with particular hereditary forms of colorectal cancer (e.g., Lynch Syndrome).