I AM AN ONCOLOGIST in the Seattle area and have a special interest in breast cancer.

Many of my patients have recently been asking me what it means to have dense breasts.

They sometimes carry a letter saying that their mammograms showed heterogeneously (or extremely) dense breast tissue.

The U.S. government now mandates that radiologists report breast density on mammogram reports but offers no guidance on how to do so.

In this Breast Cancer Awareness month, I want to provide some information about the implications of having dense breasts.

Breast Density Reporting

Beginning September 10, 2024, the U.S. government mandated this:

“Mammography facilities must provide all patients receiving a mammogram with one of two Federal breast density notification statements (either “not dense” or “dense”).“

Additionally, the mammogram report sent to referring providers must include an assessment of the patient’s breast density.

Defining Breast Density

Dense breasts are a mammographic finding in which the breast tissue appears more glandular or fibrous relative to fatty tissue.

The breast comprises two types of tissue: glandular tissue, which produces milk, and fatty tissue, which provides support and cushioning.

Dense breasts contain more glandular and fibrous tissue and less fatty tissue, which makes them appear denser on a mammogram.

Four Categories of Breast Density

The American College of Radiology classifies breast density into four categories:

1. Almost entirely fatty: The breast comprises fatty tissue and is the easiest to read on a mammogram.
2. Scattered areas of fibroglandular density: Although some areas of glandular and fibrous tissue exist, the breast is still considered primarily fatty.
3. Heterogeneously dense: There are many glandular and fibrous tissue areas, and the breast is considered moderately dense.
4. Extremely dense: This means the breast is very dense, with a lot of glandular and fibrous tissue.

Patient Notifications

A radiologist — a doctor who reads your mammogram — assigns your breasts a density.

As noted above, s/he will place each mammogram into one of the four breast density categories.

Your mammogram report may offer these observations:

• If NOT DENSE: “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 not dense. Talk to your healthcare provider about breast density, risks for breast cancer, and your situation.”
• If DENSE: “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 and a mammogram may help find cancers. Talk to your healthcare provider about breast density, risks for breast cancer, and your situation.”

Let’s turn to the implications of breast density.

Breast Density Distribution

The American College of Radiology (ACR) observes the following population distribution of breast density:

About 10 percent have almost entirely fatty breasts. At the other extreme, 10 percent have extremely dense breasts.

Eighty percent fit into one of the middle categories.

What causes dense breast tissue?

It’s unclear why some women have dense breast tissue while others do not. You may be more likely to have dense breasts if you:

• Are younger. Breast tissue often becomes less dense with age.
• Have a lower body mass index. Individuals with less body fat are more likely to have dense breast tissue than women who are obese.
• Take hormone therapy for menopause. Those taking combination hormone therapy to relieve signs and symptoms of menopause are more likely to have dense breasts.

Breast Density Implications

Having dense breasts may increase your chances of getting breast cancer.

Moreover, dense breasts make it more challenging for radiologists (and their artificial intelligence friends) to spot cancer on mammograms.

Unfortunately, dense breast tissue appears white on a mammogram.

Lumps, both cancer and non-cancer, often appear white, too. I often say to my patients that dense breasts make it like you are looking for a snowball in a snowfield.

In summary, dense breasts can raise cancer risk and make mammograms less accurate.

Mammograms Are Still Important

First, while many of my patients with dense breasts initially think that mammograms are useless for them, mammograms are still a part of routine imaging.

After all, mammograms are the only medical imaging screening test proven to reduce breast cancer deaths.

Each week, my radiology team finds cancers on mammograms in women with dense breasts.

What Else Can You Do?

That said, here are some tactics you can use:

• Get digital breast tomosynthesis (DBT or 3-D mammograms). DBT creates breast images in slices from many angles, making abnormalities easier to see. Tomo mammograms increase the number of cancers discovered without other imaging.
• Consider ultrasound and magnetic resonance imaging (MRI). These advanced imaging approaches can help find cancers that a mammogram doesn’t indicate. However, breast MRI and ultrasound show many more findings that aren’t cancer (false positive), resulting in added testing.
• Know your breast cancer risk. We have many calculators (varying in complexity and weighing different factors — including family history).

There is no national standard for following up on dense breast reports.

Screening Recommendations

The American College of Radiology suggests that you talk with your healthcare provider and discuss your breast cancer risk.

Even if you have a low breast cancer risk (and have fatty breasts), you should get annual mammograms beginning at 40.

The U.S. Preventative Task Force recommends that all women get screened for breast cancer every other year, starting at age 40 and continuing through age 74.

The Real Problem

Radiologists must describe breast density on your mammogram report.

However, we don’t know what to do with this information.

For example, the American College of Radiology and the National Comprehensive Cancer Network recommend that women and their doctors consider supplemental screening.

On the other hand, the American College of Obstetricians & Gynecologists and the U.S. Preventive Services Task Force offer there is insufficient evidence of benefit to recommend additional tests.

Additional tests can detect more cancers than mammograms alone. However, we have no evidence that extra testing saves lives.

And This

Some studies raise concerns about the reliability of the system used to label breasts as dense or not dense.

A review article in Annals of Internal Medicine reported that radiologists reclassified up to 19 percent of women into a different breast density category — from dense to non-dense or vice versa — on their subsequent screening mammogram.

For those with dense breasts, we don’t know with certainty what the best practice is at this point.