source: Canadian Cancer Society
Most colorectal cancers occur randomly or by chance (sporadically). Although 20%–30% of people with colorectal cancers have a family history of this cancer, less than 10% of all colorectal cancers are due to a hereditary or inherited gene mutation. Familial adenomatous polyposis (FAP) is the second most common hereditary colorectal cancer syndrome, but it is rare. FAP occurs in only about 1 in 10,000 people and accounts for less than 1% of all colorectal cancers.
People with FAP develop large numbers (hundreds to thousands) of polyps (called adenomas). Most polyps develop on the lining of the colon and rectum, but they can also develop in the stomach and small intestine. People with FAP usually develop polyps before age 40. Most people with FAP have a history of colorectal polyps and cancer. If the polyps aren’t treated, almost all people with FAP will develop colorectal cancer.