Researchers looked at more than 2 million mammograms that had been performed on women ages 40 to 74 at health clinics across the United States between 1994 and 2009.
About 180,000 of the mammograms were false positives, meaning that the test detected a tissue abnormality but an additional imaging or biopsy test did not support a cancer diagnosis and the women did not develop breast cancer in the year after the mammogram. The remaining tests were true negatives because they did not detect an abnormality and were not associated with the development of breast cancer in the following year.
The researchers found that an additional one woman out of 100 who had a false positive mammogram and followed it up with another imaging test went on to develop breast cancer in the next decade, as compared with women who did not have a false positive result. Among women who had a false positive followed by a biopsy, an additional two out of 100 developed breast cancer. The larger increase in risk in the biopsy group was presumably because these women had tissue that was more abnormal and thus concerning enough for the radiologist to order the more invasive follow-up test.
"Given what we found, I would say that having a false positive (result) definitely does increase your risk for developing breast cancer," said Louise M. Henderson, an assistant professor of radiology at the University of North Carolina School of Medicine in Chapel Hill. Henderson is lead author of the study, which was published on Wednesday in the journal, Cancer Epidemiology, Biomarkers & Prevention.
A modest risk factor
However, the findings do suggest that a false positive test is a relatively modest risk factor, on par with age, race or breast density, and does not increase risk as much as mutations in the BRCA1 and BRCA2 genes
, for example, Henderson said.
"Any increase in risk is worrisome for women, but our findings don't indicate that women need to be overly concerned because the absolute risk is small," she added.
Henderson recommends that women who have had a false positive test discuss it with their doctor, along with any other risk factors they might have. However she does not think the study is grounds for changing guidelines on how often women get screened.
The American Cancer Society recently updated recommendations to say that women 45 to 54 have a mammogram every year
, and women 55 and older have mammograms between once a year and every other year. Women 40 to 44 should talk with their doctors about whether to have annual mammograms.
"This study confirms findings from several international studies conducted over the past decade or so that show this association between having had a false positive mammogram with a higher risk of developing breast cancer in the following five to 10 years. I think we can now state with confidence that (it) is in fact a risk factor for developing breast cancer," said Dr. Richard Wender, chief cancer control officer for the American Cancer Society. Wender was not involved in the research.
Although the researchers did not address why a false positive result would increase breast cancer risk, they suggest it may not actually be because the abnormal tissue that the original mammogram picked up were actually cancers. An alternate explanation, Wender said, is that this suspect tissue is probably the result of some biological change, such as cells growing more rapidly, which could put predispose women to developing a true cancer down the road.
Wender agrees with Henderson that it would not make sense to recommend women with a history of a false positive mammogram get screened more often. More frequent screening is only advisable for women at risk of aggressive breast cancers, such as those with BRCA1 or BRCA2 mutations, he said. There is currently no evidence that breast cancers that arise among women who had a false positive result are any more dangerous than those among other women.
However, for women with a history of a false positive mammogram, "it becomes all the more important to stay on schedule for mammography and not put it off," Wender said.
"Some women who have a scare through a false positive actually are less likely to want to have mammograms because it was a scary experience," Wender said.
Women should take comfort in knowing the increase in risk is small, and if they do develop breast cancer and it is detected early, it should be treatable, he added.