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DNA Test for Colon Cancer a Mixed Bag

Pricey test beats blood screen, but falls short of colonoscopy

WEDNESDAY, Dec. 22, 2004 (HealthDayNews) -- A test that uses DNA to detect colorectal cancer is better than the most commonly used noninvasive screening, but a new study finds it still falls far short of the most accurate -- albeit intrusive -- method.

A comparison of a fecal DNA test with the standard fecal occult blood test found that more cancers were detected using the DNA screening. However, neither test found more than 41 percent of the invasive cancers and benign polyps.

In addition, the cost of the fecal DNA test is much higher than the occult blood test, and approaches the cost of a colonoscopy.

The report appears in the Dec. 23 issue of the New England Journal of Medicine.

While colonoscopy remains the gold standard for detecting colon cancer, the search for an accurate, noninvasive test to detect the disease continues. Many patients, cringing at the very thought of a colonoscopy, don't submit to one -- less than 40 percent of people aged 50 and older undergo screening -- so researchers are seeking a less uncomfortable way to find the disease. When detected early, colorectal cancer is treatable, yet it kills more adults than any other cancer except for that of the lung, according to the report.

"This [DNA] test may be a suitable noninvasive screening test for colorectal cancer," said lead author Dr. Thomas F. Imperiale, a professor of medicine at the Regenstrief Institute in Indianapolis.

"In a head-to-head comparison with the most widely used noninvasive screening test for colorectal cancer, a DNA test based at looking at feces was more sensitive in detecting colorectal cancer than the standard test," he added.

In their study, Imperiale's team had 4,404 asymptomatic patients supply samples for analysis by standard Hemoccult II fecal occult blood test and a fecal DNA screening that looked for 21 gene mutations as indicators of colon cancer. The patients also had colonoscopies.

The DNA test found 40.8 percent of the invasive cancers and benign polyps, while the fecal occult blood test found 14.1 percent. Moreover, the DNA test found 18.2 percent of the advanced cancerous or benign polyps, compared with 10.8 percent found by the fecal blood test.

Compared with the findings of colonoscopies, the DNA test had a specificity of 94.4 percent, compared with 95.2 percent for the fecal blood test, the researchers found. Specificity is a measure of the percent of false positive results; a high specificity means fewer false positives.

Imperiale believes the DNA test may become practical. If the cost were lower, it would replace the occult blood test now, he said. "The cost of the test is what is going to limit its use," Imperiale added.

In addition, as the technology improves, the test will get better at identifying polyps, Imperiale said: "The biggest thing is going to be, 'Can it detect lesions, and what does it cost?'"

Imperiale said this DNA test might find its place as a method of testing for fast-growing cancers during the recommended 10 years between colonoscopies. "There is no one best test for colorectal cancer," he said. "This test needs further study, and patients should be aware it may be a useful screening technique down the line for average-risk people."

The maker of the DNA screening test, Exact Sciences Inc., of Marlborough, Mass., provided grants for the study and has paid Imperiale consulting fees, according to the report.

"These data are a little disappointing," said Dr. Durado Brooks, director of colorectal cancer at the American Cancer Society. Other studies had shown better results for both tests, he added.

Ultimately, this DNA test could have some value, Brooks said. "Right now, it remains open to a lot of questions," he said. For example, to accurately detect cancers, the test would have to be done every couple of years, which makes this test very expensive compared to fecal occult blood testing, flexible sigmoidoscopy and colonoscopy.

"Right now this is an interesting technology that is still experimental," Brooks said. "It is not recommended by the American Cancer Society or by any of the other major organizations that issue screening guidelines for colorectal cancer."

More information

The American Cancer Society can tell you more about colorectal cancer.

SOURCES: Thomas F. Imperiale, M.D., professor, medicine, Regenstrief Institute, Indianapolis; Durado Brooks, M.D., director, colorectal cancer, American Cancer Society, Atlanta; Dec. 23, 2004, New England Journal of Medicine
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