April 6, 2012
False-positive Mammograms Linked To Higher Cancer Risk
Women who receive “false-positive” mammography results may be at higher risk of breast cancer later in life, a new study suggests.
The Danish study of more than 58,000 women found that those with mammogram results initially suggesting breast cancer, although none was present, had a 67 percent greater risk of developing breast cancer compared with women with negative mammograms.
To determine if women who test false-positive after mammography screenings have a higher risk of developing breast cancer than those who test negative, the researchers gathered data from a long-standing population-based screening mammography program in Denmark from 1991-2005.
The team, led by Dr. My von Euler-Chelpin at the University of Copenhagen, evaluated the risk of breast cancer and ductal carcinoma in situ in women who had received false-positive test results between the ages of 50-69. The age-adjusted relative risk of breast cancer for women who had tested false-positive for breast cancer was then compared to women who had tested negative.
The analysis showed that women who had tested negative for breast cancer had an absolute cancer rate of 339/100,000 person-years at risk, compared to women who tested false-positive, who had an absolute cancer rate of 583/100,000 person-years at risk.
The relative risk of breast cancer in women with false-positive tests was statistically significantly higher than women who tested negative -- even at 6 or more years after the test.
However, the researchers caution that "the excess breast cancer risk in women with false-positive tests may be attributable to misclassification of malignancies already present at the baseline assessment.”
"The biopsy rate is higher here than in places like Denmark," she said.
"We have no tolerance for uncertainty or missed diagnoses."
Fortunately, the rate of false-positive results is declining with the advancement of diagnostic technologies.
"Mammography is just one tool in the shed," said Weiss.
"One thing this study shows is that additional diagnostic tools need to be used in a careful way to help identify those women who would benefit from a biopsy. We need to give women the benefit of early detection while avoiding both false positives and false negatives."
Weiss said the Danish study underscores the importance of routine checkups after a false positive.
"Women who had a false positive still need to be followed carefully over time," she said.
"Even though the last thing you want to do is go back for another mammogram."
The study was published April 5 in the Journal of the National Cancer Institute.
Meanwhile, a separate study published this week finds that between 15 and 25 percent of breast cancers found by mammograms would not have caused any problems during a woman's lifetime, although these tumors were being treated anyway.
Early breast tumors are typically surgically removed soon after they are found, and are sometimes treated with radiation or chemotherapy since there are no ways to definitively determine which tumors are dangerous and which ones are not.
"When you look for cancer early and you look really hard, you find forms that are ultimately never going to bother the patient," said Dr. Gilbert Welch of the Dartmouth Institute for Health Policy and Clinical Practice, who was not involved in the study.
"It's a side effect of early diagnosis,” he told the Associated Press (AP).
The study is only the latest to examine over-diagnosis from routine screening mammograms -- when tumors are found that would only grow slowly or not at all, and would never cause symptoms or death.
The researchers used the staggered decade-long introduction of a mammographic screening program in Norway beginning in 1996. This allowed them to compare the number of breast cancers in counties where screening was offered with those in areas that had not implemented the program.
Their analysis also included a ten year time period before mammograms were offered.
The scientists estimated that one death from breast cancer would be prevented for every 2,500 women offered screening. However, six to 10 women would be over-diagnosed and treated.
Study leader Dr. Mette Kalager, a breast surgeon at Norway's Telemark Hospital, and other experts said that women need to be better informed about the possibility that mammograms can detect cancers that will never be life-threatening.
Unfortunately, doctors do not currently have a reliable way of discerning which tumors will or won´t be dangerous.
"Once you've decided to undergo mammography screening, you also have to deal with the consequences that you might be over-diagnosed," Kalager told AP.
"By then, I think, it's too late. You have to get treated."
Kalager and her colleagues only looked at invasive breast cancer, not ductal carcinoma in situ (DCIS), an earlier stage cancer confined to the milk duct.
Under the Norway screening program, mammograms were offered every two years to women ages 50 to 69. Researchers analyzed 40,000 breast cancer cases, including 7,793 that were detected after routine screening began.
They estimated that between 1,169 and 1,948 of those women were over-diagnosed and received unneeded treatment.
"The truth is that we've exaggerated the benefits of screening and we've ignored the harms," said Dr. Welch, adding that the problem of over-diagnosis has been long recognized with prostate, thyroid, lung cancer and even melanoma.
"I think we're headed to a place where we realize we need to give women a more balanced message: Mammography helps some people but it leads others to be treated unnecessarily."
The findings were published Tuesday in the Annals of Internal Medicine.