A new federal rule takes effect Tuesday that could potentially help more women spot breast cancer earlier. The FDA will require mammogram providers across the country to notify women if they have dense breast tissue, and recommend they consult with their doctor about whether they need additional screening.

While many women are unaware of their breast tissue density, about 50% of women getting a mammogram have dense breasts. It’s a factor that increases their risk of breast cancer and makes it to harder to spot cancer on a mammogram.

Here’s what all women need to know about dense breast tissue and what to do with this new information.

What are the new rules?

As of Tuesday, all mammogram reports sent to patients in the U.S. will be required to make a note of whether a woman has dense breasts. Some states, including Georgia, already have existing laws requiring women be notified of their breast density.

All mammography centers in the U.S. must comply with this rule by Tuesday, but the American Cancer Society notes many are already doing so.

What does it mean to have dense breasts?

Breast density is a measure of how much fibrous and glandular tissue there is in your breast, as compared to fat tissue. It isn’t related to breast size or firmness.

What will your mammogram report say about density?

The summary of the mammogram reports given to patients will be described as either “not dense” or “dense.”

While the exact wording can vary slightly, if your breast tissue is dense, your report will say: “Breast tissue can be either dense or not dense. Dense tissue makes it harder to find breast cancer on a mammogram and also raises the risk of developing breast cancer. Your breast tissue is dense. In some people with dense tissue, other imaging tests in addition to a mammogram may help find cancers. Talk to your health care provider about breast density, risks for breast cancer, and your individual situation.”

Why is breast density important?

Breast density is important for two main reasons:

1. Dense breast tissue makes it harder for radiologists to see cancer on mammograms. Dense breast tissue looks white on a mammogram. Breast masses and cancers can also look white, so the dense tissue can make it harder to see them. In contrast, fatty tissue looks almost black on a mammogram, so it’s easier to spot a tumor that looks white if most of the breast is fat tissue.

More than 33% of breast cancers are missed by mammography in women with dense breasts, according to estimates by George Washington University Hospital.

2. Women who have dense breast tissue have a higher risk of breast cancer compared to women with less dense tissue, according to ACS. It’s unclear at this time why dense breast tissue is linked to cancer risk. It may be that it has more cells that can develop into abnormal or cancerous cells.

The more dense your breasts are, the higher your risk, according to the Centers for Disease Control and Prevention. The level of risk is thought to be similar to the risk of breast cancer in someone who has a relative who had the disease, according to the Mayo Clinic.

Margie Singleton, started a nonprofit to help raise awareness of the importance of knowing their breast density and taking action to protect themselves from breast cancer. She was diagnosed with an aggressive form of breast cancer not detected on a mammogram. (Above) Singleton is getting her last chemotherapy treatment. Courtesy

How do you know if you have dense breasts?

Radiologists are doctors who “read” mammograms and other types of imaging tests. They check your mammogram for abnormal areas, and they also look at breast density.

Is there more for you to know?

Doctors break down breast density into four categories from almost all fatty tissue (category A) to scattered areas of dense tissue (category B) to heterogeneously dense (category C) to extremely dense (category D).

Mammogram reports sent to health care providers must include an overall assessment of breast density using the four categories. The summary report given to patients may not specify the level of density.

But if you have access to your report through a patient portal or other means, you can often find this within the report — usually near the beginning of the report. If you do not have access to the report, you can ask your provider for this information.

A woman receives her routine yearly mammogram at Mount Sinai Hospital in Chicago. A new federal rule requires women be informed if they have dense breast tissue. Heather Charles/Chicago Tribune/TNS

What should you do if you have dense breast tissue?

If your mammogram report says that you have dense breast tissue, talk with your health care provider about what this means for you, and whether you should get additional tests.

Some studies have suggested 3D mammography might be particularly helpful in women with dense breasts.

A 3D mammogram uses computers to combine X-ray pictures of the breast into a 3D image, which allows health care professionals to see past densities better. It can be used as a screening test along with or instead of a standard mammogram, although it isn’t yet available at all imaging centers.

Studies have also shown that breast ultrasound and possibly magnetic resonance imaging (MRI) can also help find some cancers that can’t be seen on mammograms. But ultrasound and MRI can also lead to false-positives that are not cancer. That in turn can lead to more tests and unnecessary biopsies.

How much will extra screenings cost?

The cost of an ultrasound or MRI may not be covered by insurance for women with dense breast tissue.

Medical associations have issued statements on dense breasts but so far have not issued clear advice on what doctors and women should do with the new density information.

The lack of guidelines means whether a woman with dense breast tissue gets additional screening and whether her insurance will pay often comes down to whether a woman has other risk factors such as a family history of breast cancer and whether her doctor writes a medical order for more tests.