False-Positive Mammograms
According to a report published in the October 18 issue of the Journal of the National Cancer Institute, almost a quarter of all women who regularly underwent screening mammograms over a 10-year period received false-positive results from at least one of their exams.
A false positive result from a mammogram means that additional testing after an abnormal result found that a woman did not actually have cancer. More specifically, it means that no cancer was diagnosed within one year after the abnormal exam.
Dr. Cindy Christiansen of Boston University and colleagues reviewed the medical records of 2,227 women who received between one and nine mammograms over the ten-year study period. They found that 6.5 percent of the total of 9,747 mammograms were false-positive. However, eventually 23.8 percent of the women had at least one false-positive result because of undergoing multiple screenings.
Importantly, the researchers identified seven specific variables that could impact the chances of a false-positive result on a screening mammography. They found that the risk of a false-positive mammogram:
Regarding the last point, among the 93 radiologists included in the study, about half had false-positive rates of less than 5 percent; about one quarter had rates between 5 percent and 10 percent; and another quarter had false-positive rates greater than 10 percent.
Nonetheless, the authors pointed out that their findings should not deter women from having mammograms, but rather should serve to educate them about their risk of a false-positive result and to reduce their anxiety when a mammogram requires special follow-up testing.
These results are in line with a similar report published previously in the New England Journal of Medicine, which found that over a 10-year period almost one out of three women who had screening mammograms and clinical breast exams had at least one false-positive mammogram.
According to research by Dr. Joann Elmore of the University of Washington School of Medicine and colleagues, 23.8 percent of the 2,400 women participating in their study had at least one false-positive mammogram over the period; 13.4 percent had at least one false-positive clinical breast examination result; and 31.7 percent had at least one false-positive result from either test. They based their findings on a review of 9,762 mammograms and 10,905 clinical breast exams, with the average woman receiving four mammograms and five clinical breast exams over the period.
Fortunately, it appears that receiving a false-positive mammogram does not deter women from undertaking subsequent mammography screening. According to a study by Dr. Marcia Burman of the Veterans Affairs Puget Sound Health Care System in Seattle and colleagues, women who have a false-positive mammogram, particularly those undergoing mammography for the first time, are more likely than others to attend their next scheduled routine breast cancer screening.
Writing in the journal Annals of Internal Medicine, the researchers collected data on subsequent mammogram behavior for 5,059 women enrolled in the Group Health Cooperative who had a screening mammogram between 1990 and 1992. Among the participating women, 813 (6.2 percent) had a false-positive result, but these women were then much more motivated to undergo subsequent screening, particularly if they had never had a mammogram before.
The authors called their findings "reassuring" in light of "the high cumulative risk for a false-positive mammogram in women who regularly undergo mammography and the importance of regular mammography in decreasing breast cancer rates."
The researchers asked the women in the study how many false positives would be acceptable for each life saved. Almost two-thirds of the women surveyed felt they could accept 500 or more false-positive results for every life saved. Just over a third of the women said they thought 10,000 false positives for one life saved was acceptable. The authors said the current best estimate of the actual number of false positives is somewhere between 30 and 200 for every life saved-well below what the majority of the women said they would tolerate. The researchers were clearly surprised at such a high tolerance level.
A small group (16 percent) of the women surveyed had actually gotten bad news from their initial mammogram and had a follow-up biopsy that did not show cancer. Even they were largely tolerant of a large number of false positives to save a single life.
Breast cancer experts universally agree that the high rate of false positives-and thus the number of unnecessary follow-up procedures such as biopsies and additional mammograms-must be reduced. However, they emphasize that women should not lose faith in the value of annual breast exams. The risk of cancer going undetected still far outweighs the short-term anxiety of receiving a false-positive test result.
SOURCES:
Journal of the National Cancer Institute, October 18, 2000; 92:1657-1666
Annals of Internal Medicine, July 6, 1999; 131:1-6, 60-61
British Medical Journal, June 17, 2000; 320:1635-1640
New England Journal of Medicine, April 16, 1998; 338(16):1089-1096
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