Cause and Risk Factors for Colorectal Cancer
The cause of colorectal cancer is unknown. Age is the primary risk factor. Incidence of the disease increases significantly after the age of 50.
Some people have a genetic predisposition to carcinogenic (i.e., cancer causing) effects of the diet on the digestive tract, which increases the risk for colorectal cancer. When this predisposition is combined with a high dietary intake of fat and red meat and a low dietary intake of fiber (e.g., bran), vitamins (e.g., folate), and minerals (e.g., calcium), the risk is even higher.
Ashkenazi Jews have a higher incidence of a specific genetic mutation (called I1307K) that increases the risk for colorectal cancer.
Lifestyle factors that increase the risk include the following:
- Daily alcohol use (may double the risk)
- Eating a high-fat, low-fiber diet
- Sedentary lifestyle
Recent studies have shown that a high intake of dietary fiber may not significantly reduce the risk for colon cancer.
A family history of intestinal polyps or colorectal cancer (especially before the age of 60) results in an increased risk for the disease. Other diseases and medical conditions that increase the risk include the following:
- Genetic disorders such as familial polyposis syndromes and hereditary non-polyposis colon cancer syndrome (HNPCC)
- Inflammatory bowel disease (e.g., ulcerative colitis, Crohn's colitis, granulomatous colitis)
- Personal history of intestinal polyps or colorectal cancer
Familial polyposis syndromes are rare genetic (i.e., inherited) conditions characterized by early onset of multiple intestinal polyps (called polyposis) and a very high (virtually 100%) risk for colorectal cancer. They also increase the risk for other types of cancer (e.g., thyroid cancer), noncancerous growths (e.g., sebaceous cysts), and osteomas (i.e., benign bone tumors).
Types include the following:
- Familial adenomatous polyposis (characterized by as many as thousands of polyps throughout the entire colon)
- Gardner's syndrome (characterized by benign tumors of the small and large intestines, lipomas [benign fatty tumors], and sebaceous cysts)
- Oldfield syndrome (characterized by sebaceous cysts, polyposis, and adenocarcinomas)
- Peutz-Jeghers syndrome (characterized by polyps throughout the digestive tract)
- Turcot syndrome (characterized by central nervous system tumors and polyposis)
Hereditary nonpolyposis colon cancer (HNPCC) syndrome is a genetic condition characterized by early-onset colorectal cancer (i.e., develops before age 50) and multiple colorectal cancers. This syndrome also may be associated with other cancers (e.g., cancer of the small intestine, endometrium, stomach, and renal pelvis).
The following criteria (called Amsterdam criteria) are used to identify patients at risk for this condition:
- At least three family members with HNPCC-associated cancer (e.g., cancer of the colon, rectum, endometrium, small intestine, renal pelvis)
- At least one of these family members must be a first-degree relative of the other two
- At least two successive generations affected
- At least one family member diagnosed before age 50
Some studies have shown that radiation to the pelvis and pelvic surgery may slightly increase the risk for colorectal cancer. However, the results of these studies are controversial.