“The ultimate test of the value of M.R.I. screening” in these women will be “whether it improves survival — an answer that we will not have for a very long time,” cautioned Dr. Dan L. Longo, a deputy editor of the New England Journal and professor of medicine at Harvard Medical School, in an editorial accompanying the study.
For all the promise, there is also a downside to using M.R.I.s for breast cancer screening: They yield many false positive results that lead to unnecessary biopsies, and they can detect very early stage tumors that might never become life-threatening, said Carla van Gils, senior author of the study and a professor of clinical epidemiology at University Medical Center Utrecht.
Nevertheless, Dr. van Gils said, the significant reduction in interval cancers — cancers that are diagnosed after a negative mammogram — suggests supplementary M.R.I.s may be a lifesaving tool for women with dense breasts.
“It’s not the same as mortality, but it’s the first step. It’s a prerequisite,” she said, adding that the study is ongoing, and mathematical models will be run to make further predictions about mortality and overdiagnosis. “There are more questions that need to be answered.”
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Only about 10 percent of women have extremely dense tissue like the women in the Dutch study. But having dense breast tissue generally makes it harder to see tumors on a mammogram because both the dense tissue and the tumors show up white on an X-ray. Fat, on the other hand, shows up black, so tumors are easily seen.
The Dutch study, called DENSE (Dense Tissue and Early Breast Neoplasm Screening), is a multi-center, randomized controlled trial of 40,373 women between the ages of 50 and 75 in the Netherlands, all of whom have extremely dense breast tissue and had a normal screening mammogram. Scientists randomly invited 8,061 of the women to undergo a supplemental M.R.I., while the remaining 32,312 had only the mammography.
Though only 59 percent of the women invited to have an M.R.I. accepted the offer and had the additional screening, 16.5 additional cancers were detected for every 1,000 women who had an M.R.I.