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Most women are aware that a family history of breast cancer increases their risk of the disease, but far fewer women understand that extremely dense breasts may pose a greater risk.

A study published Monday in the journal JAMA Network Open found that of the nearly 1,900 women who participated, the majority viewed breast density as a less important risk factor than family history.

But according to the study, women with extremely dense breasts, characterized by minimal adipose tissue, have a four times higher risk of breast cancer than women with the lowest breast densities. About 10% of women who get mammograms have this level of breast density. In comparison, having a mother, sister, or daughter who has had breast cancer is associated with double the risk of the disease.

Women who have a significant amount of dense breast tissue, but not to an extreme degree – about 40% of those who get mammograms – have a 20% higher risk of breast cancer compared to according to the study for people with an average breast density.

That’s slightly lower than the risk associated with drinking a glass of wine every night, according to Dr. Phoebe Freer, chief of breast imaging at the University of Utah Department of Radiology and Imaging Sciences, who was not involved in the study.

“Everyone has a different amount of fibroglandular tissue and a different pattern,” Freer said, referring to dense breast tissue. “It’s almost like a patient’s fingerprint.”

The only way to tell if you have dense breasts is through a mammogram, which doctors generally recommend every one or two years for women in their 40s or 50s.

The Food and Drug Administration proposed a rule in 2019 that would require mammography facilities to inform patients about their breast density and its significance. In October, the FDA said it was optimistic that the final rule would be published in early 2023.

Thirty-eight states already require health care providers to give women information about breast density after a mammogram, but not all states require health care providers to notify a woman if she herself has dense breasts.

A woman exits a mobile mammography screening bus in Anaheim, California.
A woman exits a mobile mammography screening bus in Anaheim, California, on Oct. 17, 2016.Mindy Schauer / Digital First Media/Orange County Register via Getty Images file

Because dense breasts are common, doctors may inadvertently downplay the risks, said Christine Gunn, a researcher at the Dartmouth Institute for Health Policy and Clinical Practice who conducted the JAMA study.

“There are a lot of conversations with GPs where they say, ‘This is normal.’ For some women, it translates to, ‘Oh, I don’t have to worry about that,'” Gunn said.

In individual interviews as part of Gunn’s study, six of the 61 women said that dense breasts contributed to breast cancer risk.

There are two reasons why dense breasts are linked to a higher risk of breast cancer.

First, the composition of the breast can predispose people to cancer. The reasons for this are not clear, but scientists suspect that cancer is more likely to develop in fibroglandular tissue, which is unique to the breast, as opposed to adipose tissue, which is found throughout the body.

Second, because women with extremely dense breasts have almost all fibroglandular tissue, it is more difficult to detect cancerous masses or calcium deposits on a mammogram. That makes it easier for cancer to grow or spread undetected rather than being caught and treated early.

Checking for cancer in a patient with dense breasts is like looking for a white spot on a white wall, according to Dr. Melissa Durand, an associate professor in the Yale School of Medicine Department of Radiology and Biomedical Imaging.

Durand explained that both fibroglandular tissue and cancer appear white on a mammogram, while adipose tissue appears black.

“In a completely fat breast — so much and so much black on the mammogram — we can be as accurate as 98%,” she said. “But our sensitivity can get really low — as low as 30% in some studies — if you have an extremely, extremely dense chest.”

Radiologists said the ideal type of mammogram, especially for women with dense breasts, is digital breast tomosynthesis, which is often better at detecting cancer than standard mammograms.

From there, doctors may recommend an ultrasound or MRI. Women with dense breasts should probably get additional screenings every year, radiologists said.

Ultrasounds are safe and relatively inexpensive, but Freer said they can miss cancer or show false positives, which can be confusing for patients. MRIs are the most sensitive option, but for insurance companies to cover the cost, patients usually have to demonstrate additional risk factors, such as a genetic mutation or family history of breast cancer.

“The more often you get screened, the more likely you are to get a call back for additional imaging,” Freer said. “It causes some anxiety and it certainly takes time. Most patients are willing to put up with that risk to get the life-saving benefit.”

Women can’t change their breast density, but it can change with age.

“Usually younger women have denser breasts,” Durand said. “As we age, like other parts of our body, we gain more fat, so your breast tissue can get more fatty tissue in it.”

To lower the risk of breast cancer in general, doctors recommend limiting alcohol intake, exercising regularly, and eating a healthy diet. The Breast Cancer Surveillance Consortium provides an online tool to help people measure their breast cancer risk based on multiple factors, including breast density.

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