Familial adenomatous polyposis (FAP) is a rare, hereditary condition. In its classical form, hundreds to thousands of polyps are formed in the large intestine. Treatment should be timely because almost all patients will develop colon cancer from the age of 45 onwards. In a less severe form of FAP, patients may have less than a hundred polyps.
FAP is caused by a mutation in the APC gene and affects about one in 8,000 individuals, or 1% of colon cancers. Children of patients with FAP have a 50% risk of having FAP. Family members of FAP patients should undergo genetic testing to see if they also have the same pathological gene.
Lynch syndrome is another genetic syndrome caused by a mutation and accounts for about 3-5% of all colon cancers.
People with Lynch syndrome have an 80% risk of developing colon cancer. If colon cancer is found during a colonoscopy, it should be sent for testing for microsatellite instability (MSI). Children of infected individuals have a 50% risk of developing Lynch syndrome.