A woman steps forward into a mammogram machine

Nancy Cappello’s mammogram results came back normal in 2003. So she was shocked when, just six weeks later, her doctor found a ridge under her right breast. She had advanced stage three breast cancer that had spread to her lymph nodes, and her mammogram had missed it.

Cappello later found that she, along with 40 to 50 percent of women over 40 in the U.S., has "dense breasts." That's a classification that can increase the risk of breast cancer (as confirmed by a study published in the journal Radiology earlier this week) and raise the chance that a mammogram won’t detect it. Dense breasts have more fibrous or glandular tissue and less fat.

To help others and spread the word, Cappello launched a campaign in Connecticut that helped push a new law there in 2009 that requires healthcare providers to inform women if their mammogram reveals that their breasts are dense.

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Now about 35 states have passed similar laws, and about six more, plus the federal government, are considering similar legislation. This has caused confusion for many women and even many doctors, who are uncertain how to proceed with that information.

“I’m all for women having information, but there’s not evidence to support what to do,” says Karla Kerlikowske, M.D., a primary care physician and professor of medicine, epidemiology, and biostatistics at the University of California, San Francisco.

Here’s what you need to know.

Understanding Dense Breasts

Having dense breasts is common and, by itself, just a moderate risk factor for breast cancer. While about half of women receiving mammograms have dense breasts, only about a quarter of that group has an increased risk of developing cancer that won't be detected with a mammogram, says Kerlikowske.

Still, the risk is there.

The new Radiology study, for example, found that of about 108,000 women age 50 to 69 who’d been screened for breast cancer between 2007 and 2015, those with dense breasts were 1.37 times more likely to have breast cancer than those with non-dense breasts.

Those with dense breasts were also nearly three times more likely to have an aggressive form of the cancer.

The study authors did note, however, that the absolute risk of these women getting breast cancer was still low: just 5 out of 1,000 women with non-dense breasts got cancer, and 13 out of 1,000 women with dense breasts got cancer.

Dense breasts contain many more cells, connective tissue, and collagen than less dense, or “fatty” breasts. More cells means more chances for one of them to become corrupted.

Further increasing the risk, dense breast tissue can also mask cancerous lesions on a mammogram. That extra tissue is the same shade of white as lesions or tumors on an X-ray, rendering mammography on dense breasts only 62 to 68 percent effective. That’s a big dip from the 85 percent effectiveness for women with fatty breasts.

One of the problems with the new laws, says Kerlikowske, is that they don’t tell you the extent of density—just that a woman has dense breasts. The degree of density matters. The more dense your breasts, the higher your risk.

“It’s like saying, ‘I’m notifying you that you’re tall,’” says Diana Buist, Ph.D., M.P.H, a senior scientific investigator with the Breast Cancer Surveillance Consortium and director of research and strategic partnerships at Kaiser Permanente Washington Health Research Institute in Seattle.

It’s a characteristic of your physiology, she explains, but it’s unclear exactly how it might increase your chances of cancer and what you can do about that risk. 

What You Should Do

Some states recommend that women with dense breasts pursue supplemental imaging, such as breast ultrasound or magnetic resonance imaging (MRI). But even then, says Kerlikowske, it's not known whether supplemental imaging is any more effective at detecting cancers that aren't identified on a mammogram.

Ultrasound and MRI can also be costly and time-consuming, Kerlikowske says.

The more breast imaging tests, the more chances of a false positive or finding a problem that isn’t clinically important. This could eventually subject a woman to invasive and unnecessary procedures such as a breast biopsy, not to mention anxiety and worry. (See CR’s guide to different types of breast cancer screening tests for more information.)

So what should you do if you have dense breasts?

First, don’t panic. Dense breasts alone aren't enough to significantly increase your chance of developing cancer, says Kerlikowske. The most important thing is to ask whether your doctor can identify the degree of density, and then assess your overall breast cancer risk using online tools such as the Breast Cancer Surveillance Consortium Risk Calculator.

If you have dense breasts and other risk factors, such as a family history of breast cancer, pursuing supplemental imaging, such as ultrasound or MRI in addition to mammography, might be a good option.

“There’s no clear-cut strategy,” says Buist. “Be aware of your own risk, and understand where you stand on the spectrum.”