Colon cancer increases with age. It is less common before age forty, but rises sharply after fifty, and is 50 times more common between the ages of sixty-to-eighty than before forty. The lifetime risk of colon cancer in the US is about 5%, making prevention an important strategy.
Colon polyps, which arise from colonic glands, account for 95% of colon cancers. These polyps take five-to-ten years to become malignant, which allows plenty of time for detection and removal. Polyp-makers are at an increased risk of colon cancer, even after the polyps have been removed and therefore, require more frequent monitoring. Colitis also increases the risk of colon cancer by up to twenty times, and thus sufferers need to be frequently monitored regardless of their age. Family history of colon cancer is another indication to have more frequent check-ups because hereditary conditions are responsible for up to 10% of all colon cancers.
Reversible environmental and geographical factors play an important role in promoting colon cancer. City dwelling, eating diets high in fats and red meats, and consuming low fiber foods all increase the risk of colon cancer. Reduced physical activity, increased weight, smoking, and heavy drinking are other reversible factors that also increase colon cancer and therefore should be discouraged.
Because colon cancer is a worldwide, common, and deadly disease, screening is recommended for prevention and for early detection. In two recent studies published on September 19, 2013 in the New England Journal of Medicine, researchers shared the following findings:
a) Nishihara et. al followed 98,902 subjects ages 30 to 75 for 22 years. During that period, 2% or 1815 developed colon cancer and 0.5% or 474 died from it. Among patients who underwent screening colonoscopy, 40% or 726 cancers were prevented.
b) Shaukat et. al. studied 46,551 subjects between the ages of 50 and 80 for thirty years. Tests that detect hidden-stool-blood were done every one or two years for a total of twelve years. At the end of thirty years, death from colon cancer was reduced by 32% and 437 lives were saved.
To be worthy, a screening test must show more benefits than harms. The harms of colonoscopy include hemorrhage, colon perforation, and aspiration secondary to heavy sedation. The preparatory colon cleansing is harsh and can lead to dehydration, low blood pressure, and collapse. All of these complications become more common and more devastating in the elderly, which is why screening is avoided in older age.
There are several tests designed to detect hidden-stool-blood, and although the screening itself is not hazardous, the consequences of a positive test can be. For example, a 77 year-old subject was screened and found to have hidden blood in the stools. Based on that result, he underwent colonoscopy, which lead to colonic perforation and death. The source of the blood was determined to be internal hemorrhoids, a benign condition.
Balancing harms and benefits, the United States Preventive Service Task Force has issued the following colon cancer screening guidelines:
a) Screening for colon cancer should begin at age 50 with tests for hidden-stool-blood or some form of colonoscopy, and should continue until age 75.
b) Routine screening should be avoided in subjects between 76 and 85 years unless there is a good medical reason such as unexplained abdominal symptoms.
c) Routine screening should not be done on subjects older than 85.
These recommendations exclude subjects who are being followed for colon polyps. Depending on the type of polyp, colonoscopy is usually repeated every three to five years.
When the first colonoscopy is negative, a screening interval of ten years is recommended, but that interval can be shorter if there is family history of colon cancer. Screening for hidden-stool-blood should be done annually until age 75, regardless of colonoscopy results.
Whether screening colonoscopy is superior to annual hidden-stool-blood tests will be determined by studies, which are currently in progress. Taking aspirin to prevent colon cancer has not been endorsed by prospective studies and may be more harmful than helpful.
The benefits of screening outweigh the potential harms for subjects 50 to 75 years old. However, the likelihood that detection and early intervention will yield a mortality benefit declines after age 75, rendering screening potentially more harmful than helpful.