By the time a woman has had 10 mammograms, she will have a 50 percent chance of being told her results are abnormal, according to the study published in the Journal of the National Cancer Institute (Vol. 92, Issue 20) by Cindy Christiansen, PhD, and her colleagues from Boston University School of Public Health.

Some women are almost certain to be told that there is something wrong with their mammograms and then learn they do not have cancer. A false-positive mammogram means the radiologist who read the mammogram saw a suspicious change in the breast but no cancer was found when the women returned for more studies. These studies include a more thorough mammogram, a sonogram of the breast or a biopsy. In a biopsy, a small sample of tissue is removed, most often with a needle, to study under the microscope.

Dr. Christiansen and her colleagues studied the mammogram results of 2,227 women at health centers of Harvard Pilgrim Health Care, a large HMO in the Boston metropolitan area. The women received a total of 9,747 mammograms over 10 years. Their ages ranged from 40 to 80. Ninety-three different radiologists read the mammograms, and overall they diagnosed 634 mammograms as suspicious that turned out to be false positives.

The researchers found several factors may have led to the false-positive mammograms. False positives were seen more often in women who were premenopausal or who were postmenopausal and taking estrogen, women who had previous breast biopsies and women with a family history of breast cancer.

The radiologist also played a role. Some radiologists had a higher rate of false positives than others. A woman who consistently went to a radiologist with a low rate of false positives would only have a 15 percent chance of a false positive after nine mammograms. If she went to a radiologist with a high rate of false positives, she would have a 90 percent chance after nine mammograms. The chance of a false positive also increased if the radiologist didn’t compare the new mammogram with the woman’s old mammograms.

The researchers discovered that women who lacked the risk factors and went to a radiologist with a low rate of false positives would have only a 5 percent chance of getting a false-positive result after nine mammograms. Women who had high risk factors and visited a high-rate radiologist would have a 100 percent chance of a false positive by the time they had their fifth mammogram.

The American Cancer Society guidelines (ACS) recommend all women over age 40 have a screening mammogram every year, indicating that by the time a woman reaches age 50, she would have had nine mammograms.

Dr. Christiansen and her colleagues point out that "several studies have found adverse effects such as anxiety, distress and intrusive thoughts among women who have experienced false-positive mammographic readings."

Because there is no way to avoid false-positive readings, the researchers say "predicting the risk of false-positive mammograms may be an important way to educate women about screening and to deal with the occurrence of abnormal mammograms. If women understand their risks of false-positive mammograms, they might be less anxious when an abnormality is found".

Women also can lower their risk of getting a false-positive result by insisting that their new mammograms be compared to their old results, says Herman Kattlove, MD, medical editor for the ACS. For a single mammogram, using the old film for comparison reduces the false-positive rate from 8 percent to 4 percent. By the time a woman has had nine mammograms, the false-positive rate lowers to 33 percent from 54 percent when the old film is used.

"If a woman changes health plans or providers, she should make sure copies of her old records are available," Dr. Kattlove says. "Unless there is a good reason to change, it makes sense to stick with the same radiology facility year after year so that films are available. These suggestions are especially important for women with several risk factors identified in the study as contributing to a high false-positive risk, including prior biopsies, family history of breast cancer and being premenopausal."

Also, because there are substantial differences among radiologists’ false-positive rates, women should get a second opinion from a radiologist who specializes in mammography, Dr. Kattlove says.

Breast cancer is the second leading cause of cancer deaths in women. The ACS predicts that in the year 2000, 184,200 women will be newly diagnosed with breast cancer and 41,200 women will die of the disease.