The wisdom of colonoscopy screening seems obvious. The test enables a physician to examine the lining of the entire colon and to remove small, potentially precancerous growths called polyps during the exam. As a result, it has the potential not only to detect colon cancer early, but also to prevent new cases by removing polyps. It’s generally assumed that colonoscopy saves lives because the procedure is good at detecting early disease.
A report from the National Polyp Study in the Feb. 23, 2012, issue of The New England Journal of Medicine supports this assumption. The study included 2,602 people who had adenomatous polyps (the type most likely to progress to cancer) removed during colonoscopies that were ordered because of findings on other screening tests, symptoms, or a family history.
During an average follow-up period of about 16 years, 12 people in the study died of colorectal cancer, which was less than half of the 25.4 deaths from colon cancer normally expected in a group that size drawn from the general population.
Not the Final Word
However, the study has significant limitations. It’s possible that the people who had polyps removed had fewer risk factors for colon cancer than the general population. They did, in fact, have a significantly lower death rate from all causes, which suggests that they may have been healthier over all or received better health care. Most importantly, this wasn’t a randomized trial, designed to compare a screened population with an unscreened one, but a study that made use of existing data to make comparisons.
Because of these limitations, Dr. Robert J. Mayer, a senior physician at the Harvard-affiliated Dana-Farber Cancer Institute and a member of the Harvard Health Letter’s Editorial Board, says the results, while promising, can’t be the final word on colonoscopy. That may come from randomized controlled trials like the NordICC trial, now under way in several European countries where colonoscopy screening isn’t routine. The problem is that results won’t be available for another decade.
Meanwhile, colonoscopy has become the dominant form of cancer screening in this country. Some argue that the American health care system was too quick to adopt a screening test that’s invasive, expensive, and hasn’t been shown definitively to save lives. Yet colonoscopy has some clear advantages. The entire colon is visualized directly. It’s a one-stop test; all the other colon cancer screens require a follow-up colonoscopy if anything suspicious is found.
Finally, because colonoscopy entails identifying precancerous polyps and removing them if they’re found, it functions as a screening test and as a preventive intervention. Colon cancer incidence–the number of new cases–has been falling in the U.S., and screening colonoscopy very likely deserves a large share of the credit.
Considering a Colonoscopy?
When you consider getting a colonoscopy, take the following into account:
1. The benefits. Colon cancer is the fourth most common cancer in the U.S. and the second leading cause of cancer death. The risk of developing colon cancer rises at age 50, the age at which guidelines recommend screening begin for most people.
2. The risks. Colonoscopy requires a thorough bowel cleansing with a laxative the day before the test. For the test, the patient is sedated, the colon inflated with air or carbon dioxide, and a colonoscope–a thin, tube-like instrument with a light and a lens for viewing as well as a tool to remove tissue for biopsy–is inserted into the rectum and passed through the entire colon. Although the risks are slight, they may include kidney damage from sodium-phosphate laxatives, a reaction to the sedative, a perforation during the procedure, and bleeding if a polyp is removed.
3. The likelihood of a false-negative test. Colonoscopy is estimated to miss about 5 percent of colon cancers, most of them “upstream,” in the part of the colon farthest from the rectum.
Guidelines recommend colonoscopy every 10 years for people at average risk. But there are other screening methods and schedules. A test for blood in the stool can detect colon cancer at an early stage, and is supposed to be done every year. Sigmoidoscopy is similar to colonoscopy but requires far less bowel cleansing; it is recommended every five years, usually in conjunction with annual stool tests.
Randomized trials have shown that sigmoidoscopy reduces colon cancer incidence and deaths from the disease, but it visualizes only the lower part of the colon (although that is where most colon cancers occur). Double-contrast barium enemas give a view of the entire colon, require cleansing prep but not sedation, and are supposed to be done every five years. They miss more polyps than colonoscopy and involve radiation exposure.
CT scans of the colon–sometimes called virtual colonoscopy–are also supposed to be done every five years. They require the cleansing prep and inflation of the colon, but not sedation. CT scans are almost as good as colonoscopy at finding polyps, but also involve radiation exposure.
If you’re at higher-than-normal risk of colon cancer because first-degree relatives have had colon cancer or because you have had polyps, you should be advised to have a colonoscopy every three to five years instead of every 10. But if you’re older or have other major health problems, so you wouldn’t want to undergo treatment even if colon cancer were detected, you may want to forgo colon cancer screening, no matter what the test. Talk it over with your doctor.
- Harvard Health Letters
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