Colorectal cancer risk and genetics
contributed by Elena Martinez Stoffel, M.D., MPH
Everyone is at risk for colon cancer, but that risk is not the same for everyone. Colorectal cancer is the third most common cancer affecting men and women and each year, there are about 93,000 new cases of colon cancer and 39,610 new cases of rectal cancer diagnosed in the United States. The average person has a 5 percent chance of developing colon cancer, but some people are at a higher risk.
The most common risk factors for colon cancer are:
- Race/ethnic background
- Personal medical history
- Family medical history
The risks of developing colon cancer increase with age for both men and women. About nine out of 10 people diagnosed with colorectal cancer are 50 years old or older. That’s why guidelines recommend that even individuals without risk factors start colon cancer screening at age 50; individuals whose histories place them at higher than average risk may need to begin screening at younger ages (see below).
Leading a healthy lifestyle can reduce the risk of getting colon cancer. Although the effect of diet on colon cancer risk has been difficult to quantify, there is good evidence that obesity can increase your risk, as can physical inactivity, smoking and heavy alcohol consumption.
African-Americans have a higher risk of getting colon cancer compared to Caucasians, Asians or Latinos; however the reasons for these differences are not yet clear.
Personal medical history
People with a history of precancerous colorectal polyps are at increased risk for colorectal cancer. Those with type 2 diabetes or a history of inflammatory bowel disease (IBD) also have an increased risk. Over time, chronic inflammation (such as occurs in IBD) can affect the cells lining the colon or rectum and cause them to become cancerous.
Family medical history
If you have a first degree relative (parent, sibling, or child) who has had colorectal cancer, you are at an increased risk as well. Approximately one-third of colorectal cancers are diagnosed in individuals with a family history of colorectal cancer. If you have a family history of colorectal cancer, you may need to begin screening colonoscopies earlier than most people. The general rule for those with a family history is to have the first colonoscopy 10 years earlier than the age of diagnosis of your family member or by age 40, whichever is younger. So, if your family member was diagnosed with colon cancer at age 35, you should have your first screening at age 25.
Hereditary cancer syndromes account for 3 to 5 percent of all colon cancers, with many of these diagnosed at young ages. Lynch syndrome is one of the most common hereditary cancer syndromes and is associated with genetic predisposition to colorectal cancer -- up to an 80 percent lifetime risk in the absence of colonoscopy. Tumors from patients with Lynch syndrome have very specific characteristics, and guidelines recommend that all colorectal cancer tumors be screened for those characteristics. A diagnosis of Lynch syndrome can be confirmed with genetic testing and if an individual is diagnosed with Lynch syndrome it is possible to offer genetic testing to other family members so they can start screening early. Individuals with Lynch syndrome should begin colonoscopies at age 20 to 25 with repeat exams every one to two years -- this can dramatically reduce the risk of getting colon cancer.
FAP (familial adenomatous polyposis) is another genetic condition associated with high risk of developing colorectal cancer. Individuals with FAP develop hundreds of colon polyps and should start having colonoscopies at age 10 to 12.
Individuals with a family history that suggests possible inherited risk for cancer should work closely with healthcare professionals to monitor their health and take steps to prevent cancer. It’s also important to share this information with family members so everyone can have their own risks assessed.