Colon cancer is a malignant tumor of the large intestine. It affects 2 percent to 6 percent of all men and women in their lifetime. It is the third most common cancer in both sexes.

Most colon cancer cases are not hereditary. Only about 5 percent of people with colon cancer have a hereditary form. In those families, the chance of colon cancer is much higher than for the average person. Identifying people and families that might be at-risk for hereditary colon and associated cancers can dramatically reduce the number of cancer diagnoses in these families.

Colon Cancer Genes

Several genes which contribute to making a person susceptible to colon cancer have been identified. The two most common inherited colon cancer conditions are FAP (familial adenomatous polyposis) and HNPCC (hereditary nonpolyposis colorectal cancer), called Lynch syndrome.

• FAP (familial adenomatous polyposis): People with this syndrome develop many polyps in their colon (often over 100). People who inherit mutations in the APC gene have a nearly 100 percent chance of developing colon cancer by age 40. Also, having FAP increases the risk of developing hepatoblastoma, dermoids, fibromas, and other cancers. If a person has more than 10 adenomatous polyps in their lifetime, a cancer risk assessment is appropriate.
• Lynch syndrome (HNPCC): People with an HNPCC gene mutation have an estimated 80 percent lifetime risk of developing colon or rectal cancer. There is also a 40 percent to 60 percent chance for endometrial cancer. Other cancer risks are increased as well.

Who Should Get a Cancer Risk Assessment

These patients with the following characteristics should be referred for a cancer risk assessment:

• Diagnosed with colon cancer younger than age 50 years
• Multiple colon cancers or more than one HNPCC-related cancer*
• Colon cancer and one relative with an HNPCC-related tumor* under age 50 years
• Colon cancer and two or more first- or second-degree (parents, siblings, aunts, uncles, grandparents) relatives with HNPCC-related* cancers at any age

*Colon, endometrial, ovarian, stomach, small bowel, biliary tract or transitional cell of the renal pelvis.

Testing for Colon Cancer

Gene testing can identify some people who carry genes for FAP and some HNPCC cases of colon cancer.

But, the tests are not perfect at this point in time. So, some families may have alterations in the FAP or HNPCC gene that cannot be detected. Though FAP and HNPCC are the most common hereditary causes of colon cancer, there are other genes that may also contribute to a family history of colorectal cancer or polyps.

Recommendations for Hereditary FAP or HNPCC

FAP can start in childhood. Screening for polyps begins between ages 7-12. Some doctors even advise screening for liver tumors starting at birth. Therefore genetic risk assessment is done in childhood.

Patients with Lynch syndrome or strongly suspected Lynch need to have colonoscopy exams starting at age 20-25, and this needs to be repeated every one to two years. Also, screening for endometrial cancer is advised and includes transvaginal ultrasound and endometrial biopsy. This most often started between the ages of 35-40 and done each year.

Some patients have surgery to reduce their cancer risks. Careful screening can increase early detection of subsequent cancers. These cancers most often start young so screening must start before age 50. Though some of these options may reduce the risk for developing colon cancer, no option totally get rids of this risk.