Which Colon Cancer Screening Test Should I Get?

Q: I'm 50 and have never been screened for colon cancer. Which test should I have - sigmoidoscopy or colonoscopy?

A: This question is under debate by primary care clinicians, health insurers, gastroenterology specialists, and the government agencies responsible for making screening recommendations. At present, no single test is officially recommended for colon cancer (also called colorectal cancer), but all experts agree that women and men should begin screening by age 50. Colon cancer is the second-leading cause of all cancer deaths in the United States. Women and men are at equal risk, although risk increases with age and a high-fat diet, and is higher for anyone who has a first-degree relative with the condition. Studies have shown that detection and early treatment of a precancerous polyp - a growth on the inside surface of the colon (large intestine) or rectum - or a cancerous lesion can substantially reduce the risk of developing or dying from colon cancer.

Three tests are commonly used:

    * The fecal occult blood test (FOBT) detects traces of blood in the stool that might come from a polyp or early colon cancer. Usually, the amount is so small that it's invisible to the naked eye. Using a home kit, you collect three samples of stool and return them to your clinician's office for analysis. Studies have shown that regular testing of stool for occult blood can reduce the risk of dying from colon cancer by 30% - although some experts think this success rate is partly due to the chance discovery and removal of polyps during colonoscopies to follow up falsely positive FOBTs.
    * Sigmoidoscopy is a procedure in which the physician inserts a flexible lighted tube into the rectum to look directly at the last two feet of the colon, where cancer most often occurs. The test requires preparation with an enema, but not sedation or a hospital stay. Most people find sigmoidoscopy uncomfortable, but it usually takes less than 15 minutes. Sigmoidoscopy lowers the risk of dying from cancer on the left side of the colon by more than 50%.
    * Colonoscopy is similar to sigmoidoscopy, except that the doctor uses a longer scope to look at the entire large intestine. The preparation, which involves laxatives and an enema, is more extensive than for sigmoidoscopy, and the procedure usually requires a sedative. If a suspicious lesion is detected, a biopsy is taken and sent to the laboratory for analysis. If a polyp is found, it can usually be removed during the colonoscopy itself.

If an FOBT is positive or a sigmoidoscopy shows something abnormal, your doctor will refer you for a follow-up colonoscopy.

Most clinicians advise screening via one of two methods, beginning at age 50, for people of average risk:

    * Sigmoidoscopy every 5 years and an annual FOBT.
    * Colonoscopy every 10 years without FOBTs or sigmoidoscopy. If there is a very strong family history, experts advise more frequent screenings starting at an earlier age.

Some clinicians prefer colonoscopy for screening because it looks at the entire large intestine. But so far, neither has been shown to be superior in lowering the risk of colon cancer death.

Colonoscopy does have a greater - albeit very small - risk of complications, such as bowel perforation. It's also more expensive, costing anywhere from $200 to more than $2,000. Most insurance companies pay for sigmoidoscopy and FOBTs, but few cover colonoscopy for routine colorectal cancer screening. However, in July 2001, Medicare recommended that its carriers pay for screening colonoscopy and other insurance carriers may follow. In the years ahead, official screening recommendations may swing more toward colonoscopy.

A new technology you may have read about is virtual colonoscopy, which looks at the colon using computed tomography (CT) and virtual reality computer software. This procedure doesn't demand the prolonged insertion of a testing instrument into the colon, so there's less discomfort and no need for sedation. But it does require preparation with laxatives, and there is some discomfort when the colon is inflated with air for the examination itself.

As yet, this virtual procedure does not identify small cancerous or precancerous growths as well as colonoscopy does. And if polyps are found, a colonoscopy is still necessary to remove them. Nonetheless, with some refining, virtual colonoscopy might prove very useful, particularly if it permits physicians to screen large numbers of people at low cost.

Last updated Nov 26/06