A CRITICAL LOOK AT COLON CANCER TESTING

The recent colon cancer diagnosis of baseball player Daryl Strawberry has spawned a flurry of media stories bemoaning the lack of attention given this disease. The media reports invariably overstated the value of early detection and advised older people to undergo screening tests. Few mentioned prevention despite a large body of evidence showing that many cases of colon cancer can be attributed to a high-fat, low-fiber diet. Screening tests, by definition, are given to people without symptoms in the hope of finding a disease at an early, more curable stage. Since most screening tests carry risks like misdiagnosis and unnecessary surgery, they should be undergone only after safety and efficacy have been thoroughly proven. Unfortunately, this is not the case with colon cancer.

There are major contradictions in the message to the public about the value of screening for colon cancer. The American Cancer Society (ACS), the most frequently cited source in the media, recommends that testing begin at age 50 and at age 40, if theres a family history of colon cancer. On the other hand, the National Cancer Institute is spending millions of dollars on an ongoing clinical trial, which is trying to determine whether testing people for early colon cancer will result in a lower death rate. In this nationwide trial, men and women over the age of 55 are randomly assigned to be tested or not tested*.

If it were firmly established that screening for colon cancer saves lives, then it would be unethical for the government to conduct a clinical trial in which half the participants are not periodically tested. In other words, despite decades of research, the value of early colon cancer detection is uncertain. It is also unclear which is the better screening procedure, the FOBT or sigmoidoscopy (see box at right), or both in combination. As for screenings attendant risks, false-positive results (erroneous evidence of cancer) can lead to additional invasive testing or unnecessary surgery. For example, sigmoidoscopy may lead to the discovery and treatment of polyps that will never become malignant in a patients lifetime. (Autopsy studies have shown that 10-33% of older adults have colon polyps at death, but only 2-3% have colorectal cancer.)

The U.S. Preventive Services Task Force is a far more reliable source of information about screening tests than the ACS because the Task Force conducts a careful review of all scientific evidence before recommending for or against screening tests. The ACS colon cancer recommendations are: a digital rectal exam for all adults after age 40, annual fecal occult blood test (FOBT) at age 50, and sigmoidoscopy every 3-5 years beginning at age 50; people with a family history should have a colonoscopy ten years before the age at which their relative developed colon cancer.

But the Task Force, after reviewing all relevant studies made more cautious science-based recommendations: Recent studies have provided compelling evidence of the effectiveness of FOBT and sigmoidoscopy screening, but the evidence is not definitive. The Task Force report went on to state that it is not clear which of these two tests is preferable or whether the combination of FOBT and sigmoidoscopy produces greater benefits than either test alone. As for people with a first-degree relative [parent or sibling] with colon cancer, the Task Force states, It is not clear that the modest increase in the absolute risk of cancer justifies routine use of colonoscopy over other screening methods.

Just how much of a benefit will early detection of colon cancer provide? The ACSs 92% five-year survival rate typically overstates the benefit. The five-year endpoint is arbitrary and holds no significance for long-term survival. It simply means that 92% of people with early-stage colon cancer are alive at five years; it doesnt mean they are disease-free. Whats important to people considering a test is the question of how much will it reduce the odds of dying of colon cancer. The answer is more modest than the 92% survival rate implies. In the last few years, three randomized clinical trials have shown a 15-18% lower death rate among people screened with the FOBT relative to untested people ("Cancer," 5/15/98). The question of whether sigmoidoscopy can improve this rate will be answered by the National Cancer Institutes ongoing study whose results are expected in about five years.

*In a clinical trial involving screening tests, participants who form the control group are given what researchers call routine care. They are not periodically tested for colon cancer, as are the people in the study group, but they are not prevented from having tests when desired.

LOOKING FOR COLON CANCER
Digital rectal examination has limited value because fewer than 10% of all cases of colorectal cancer are within reach of the physician's examining finger.

Fecal Occult Blood Test (FOBT) for abnormal gastrointestinal bleeding can be performed at home with kit provided by your physician. Using a small applicator, smears are taken from three stool specimens and sent to a lab. False-positive results can occur for a variety of reasons, including certain foods, drugs, and bleeding caused by non-cancerous conditions like hemorrhoids and ulcers.

Flexible fiberoptic sigmoidoscopy is a long, thin tube inserted well into the colon that allows the examining physician to view up to 75% of the sigmoid colon. False-positive results can occur because the procedure identifies polyps that would never become cancerous.

Colonoscopy is a more extensive examination of the colon, which can reach 95% of colorectal cancers. But the Task Force found that there was insufficient evidence to prove the procedure's safety and efficacy as a screening test. Colonoscopy involves sedation, higher costs, in-hospital use, and greater potential for complications.

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