Screening with MRI in between regular screening mammograms may improve breast cancer detection in women with extremely dense breasts, according to a Dutch study.
Still, these MRI screenings resulted in a relatively high false-positive rate of 8%. It’s also not clear if finding these breast cancers in between regular screening mammograms leads to better survival.
When a screening test shows an abnormal area that looks like a cancer but turns out to be normal, it’s called a false positive. Ultimately the news is good: no breast cancer. But the suspicious area usually requires follow-up with more than one doctor, extra tests, and extra procedures, including a possible biopsy. There are psychological, physical, and economic costs that come with a false positive.
The study was published online on Nov. 27, 2019, by the New England Journal of Medicine. Read the abstract of “Supplemental MRI Screening for Women with Extremely Dense Breast Tissue.”
What is breast density and how is it measured?
Dense breasts have less fatty tissue and more non-fatty tissue compared to breasts that aren't dense.
One way to measure breast density is the thickness of tissue on a mammogram. The BI-RADS (Breast Imaging Reporting and Database System), which reports the findings of mammograms, also includes information on breast density. BI-RADS classifies breast density into one of four groups:
- mostly fatty
- scattered areas of density
- consistently dense
- extremely dense
Still, no one method of measuring breast density has been agreed upon by doctors. Breast density is not based on how your breasts feel during your self-exam or your doctor's physical exam. Dense breasts have more gland tissue that makes and drains milk and supportive tissue (also called stroma) that surrounds the gland. Breast density can be inherited, so if your mother has dense breasts, it's likely you will, too.
Research has shown that dense breasts can:
- be twice as likely to develop cancer as nondense breasts
- make it harder for mammograms to detect breast cancer because breast cancers (which look white like breast gland tissue) are easier to see on a mammogram when they're surrounded by fatty tissue (which looks dark)
Nearly half of women age 40 and older in the United States are classified as having dense breasts.
Because having dense breasts can increase a woman’s risk of breast cancer, experts have wondered whether women with dense breasts should have additional screening, on top of mammograms. In its 2019 guidelines, the U.S. Preventive Services Task Force said there wasn’t enough evidence to recommend that women with dense breasts have additional screening. So researchers have been trying to figure out if more screening would benefit women with dense breasts without leading to overdiagnosis.
Overdiagnosis means that a breast cancer screening test finds either:
- a suspicious area that would have been eventually diagnosed as cancer by other means, without any effect on prognosis
- a suspicious area that never would have affected a woman’s health if it hadn’t been found or treated
How this study was done
This study was done in the Netherlands between December 2011 and November 2015. In the Netherlands, women age 50 to age 75 are invited to have a mammogram every 2 years.
The study included 40,373 women age 50 to 75 with extremely dense breasts and normal results on their last screening mammogram. Of these women, 8,061 were randomly invited to have an MRI screening in between regular mammogram screening. Of the women invited to have an MRI screening, 4,783 (59%) actually had an MRI.
The researchers looked at how many breast cancers were detected in between screenings to see how effective MRI was in finding interval cancers. Interval cancers are cancers diagnosed after a normal mammogram and before the next scheduled mammogram.
The rate of interval cancer detection was:
- 2.5 per 1,000 screenings for women having both mammogram and MRI
- 5.0 per 1,000 screenings for women having just a mammogram
"We found that supplemental screening with MRI in women with extremely dense breast tissue resulted in the diagnosis of significantly fewer interval cancers than the use of mammography alone," the researchers wrote.
The idea is that adding MRI to mammograms led to the detection of cancers that the mammogram missed, which led to lower cancer detection rates during the next screening because the women likely saw their doctors about any suspicious findings.
The results suggest that among women with dense breasts, the risk of interval cancers is cut in half by having MRI after a normal mammogram.
But does finding interval cancers lead to better outcomes?
The researchers pointed out that the study was not large enough to look at the effect of MRI screening on breast cancer-specific survival or overall survival.
“The ultimate test of the value of MRI screening in women with extremely dense breast tissue will be whether its use improves survival — an answer that we will not have for a very long time,” wrote New England Journal of Medicine deputy editor Dan Longo, M.D., in an editorial that accompanies the research.
“In the meantime, we now have a trial showing that MRI screening can lead to a lower rate of interval cancers,” he continued. “The cost is that 74% of the biopsies that are subsequently performed will not lead to a cancer diagnosis, and we do not know whether the cancers that were detected needed to be found or treated.
“The findings of this trial are likely to reinforce the idea that MRI screening is important in women with dense breast tissue. But will we be putting these women at increased risk of procedures without contributing to their eventual survival?”
What this means for you
This study may seem confusing, but here is what we know right now:
- Women with dense breasts have a higher risk of developing breast cancer.
- Dense breast tissue can make it hard to see a cancer on a mammogram.
- Additional screening for women with dense breasts increases the rate of cancer detection, and these cancers are found earlier, when they are likely to be smaller and may be easier to treat.
What we don’t know right now:
- If finding these cancers earlier leads to better survival for women.
- If these cancers were never found, would they affect a woman’s health and life?
“I think most of these studies fail to capture the patient’s perspective and priorities on the issue of early detection,” said Breastcancer.org Founder and Chief Medical Officer Marisa Weiss, M.D. “For example, many women would say they would prefer early detection so they would have a lower risk of needing mastectomy or chemotherapy. That happens through early detection.
“The authors and medicine at large are appropriately concerned about overdiagnosis and overtreatment,” she continued. “But to reduce the risk of overtreatment, we need more tools to identify women who can avoid certain treatments, as well as those women who are likely to benefit from certain treatments. Until we have the perfect screening test and can better predict which cancers are lethal and which can be managed minimally, we depend on finding cancers early when they are more easily treatable with the least aggressive therapies.”
Written by: Jamie DePolo, senior editor
Reviewed by: Brian Wojciechowski, M.D., medical adviser