Choosing a screening test isn't always an easy decision, but it is a potentially lifesaving one. Colorectal cancers generally are not fast growing. They may occur anywhere along the wall of the large intestine. Most have their beginnings as a tiny polyp that develops on its inner lining. Not all polyps become cancerous, but nearly all colon cancers start as polyps.
Polyps are merely growths of tissue arising from the inner lining of the colon or rectum.
Large and flat polyps are more likely to become cancerous. Adenomas more commonly become cancerous as they grow. This is especially true when a polyp's diameter reaches 10 mm. An adenomatous polyp takes an average of 7 to 10 years to become cancerous. However, in some people at high risk of colorectal cancer such as those with a strong family history of colorectal cancer the cancerous change may occur much more quickly.