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MONDAY, Aug. 23 (HealthDayNews) -- The risk of getting a false positive result on a mammogram is far less than previously estimated, Norwegian research shows.

And the risk of having an invasive procedure based on a false positive is just over 6 percent, the researchers say.

Over a 20-year period, women have a 1-in-5 chance of having a false positive mammogram, according to the study findings, published in the October issue of Cancer.

"Women attending a biennial screening from the age of 50 until she reaches 69 years runs a cumulative risk of 20.8 percent for having a false positive recall," said study author Solveig Hofvind, a researcher at the Cancer Registry of Norway in Oslo.

And, Hofvind added, the chances of having to undergo an follow-up invasive testing procedure were even lower.

"The risk of undergoing a negative fine needle aspiration cytology was estimated to be 3.9 percent; a core needle biopsy, 1.5 percent, and an open biopsy, 0.9 percent," she said.

More than 215,000 American women are diagnosed with invasive breast cancer and more than 40,000 die each year, according to the American Cancer Society. As with most cancers, early detection is key. So, the ACS recommends that women over 40 have a mammogram every year.

But, there's been concern that the test may have a high false positive rate, which would mean that women would have to suffer through unnecessary anxiety, and possibly invasive testing as a result of the false positive.

"Women should know their risk of a false positive recall," said Hofvind, because if they know it's a possibility, having a false positive may be less psychologically distressing.

One previous study found the false positive rate to be as high as 50 percent if a woman has undergone 10 screening mammograms, according to Hofvind's study.

Another study, Hofvind reported, estimated the risk to be between 5 percent and 100 percent after nine mammograms. The researchers said the differences in these studies may have been due to variants in screening programs, the threat of malpractice, or the design of the studies.

To try to more definitively pin down the true false positive rate, Hofvind and her colleagues studied data from 83,416 Norwegian women who were between the ages of 50 and 69.

The women had screening mammograms every two years beginning in 1996, and the study results were based on the first three rounds of screening. The same equipment was used for all three screenings, and each mammogram was reviewed independently by two radiologists.

Overall, the authors estimate that one in five women who begin screening will have at least one false positive result after 20 years of screening. And, the cumulative risk of having an invasive test is 6.2 percent, with a less than one percent chance of having to undergo open biopsy.

The authors also found that the highest rates of false positives occurred on the first round of mammograms. Accuracy improved with each subsequent screening. For example, in the 50 to 51 age group, there was a false positive rate of 3.5 percent for the first screening, 2.2 percent for the second screening and 2.0 percent for the third screening.

Dr. Yelena Novik, an oncologist at New York University's Clinical Cancer Center, said this "study makes a good, positive point."

"Mammography is the best way of screening we have right now. There is a strong benefit for screening and finding early cancers," Novik said.

"For women who are concerned that mammograms will find some abnormality that requires a procedure and then turns out to be nothing, I would say the chance of that is not very high," Novik added.

More information

To learn more about mammograms, visit the American Cancer Society.

(SOURCES: Solveig Hofvind, M.Sc., researcher, Cancer Registry of Norway, Oslo; Yelena Novik, M.D., oncologist, Clinical Cancer Center, New York University, and assistant professor of medicine, New York University School of Medicine, New York City; October 2004, Cancer)

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