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What does it mean if I have dense breasts?


What is dense breast tissue?

Most medical organizations recommend that women who have an average risk of breast cancer consider regular mammogram testing beginning in the 40’s and consider repeating the test every 1-2 years. Dense breast tissue refers to the appearance of breast tissue on a mammogram. It’s a normal and common finding. It does not refer to how the breast tissue feels on exam.

Breast tissue is composed of milk glands, milk ducts and supportive tissue. These make up the dense tissue in the breast. Breasts also include fatty tissue, which is not dense tissue. Some women’s breasts are denser than others.

On a mammogram, fatty tissue appears dark and transparent. Dense breast tissue appears as a solid white area, which makes it difficult to see the contrast that make breast cancer visible.

Having dense breast tissue will not affect your daily life. However, it increases the chance that breast cancer will not be found on a mammogram. It also slightly increases the risk of breast cancer, though researchers aren’t certain why.

How do I know if I have dense breast tissue?

The radiologist who analyzes your mammogram determines the ratio of non-dense tissue to dense tissue and assigns a level of breast density. Levels of density are described using a reporting system created by the American College of Radiology called Breast Imaging Reporting and Data System (BI-RADS).

The levels of density are recorded in the mammogram report using letters:

Breast density imaging categories

A — This means that the breasts are almost entirely composed of fat. About 1 in 10 women has this result.

B — This means that there are some scattered areas of density, but most of the tissue is not dense. About 4 in 10 women have this result.

C — This means there are some areas of non-dense tissue, but most of the tissue is dense. About 4 in 10 women have this result.

D — This means that nearly all the breast tissue is dense. About 1 in 10 women has this result.

In general, women whose breasts are classified as level C or level D are considered to have dense breasts. About half of women who have mammograms have dense breasts.

Four categories of breast density

In these examples of each of the four density categories, a graphic depicting breast density is above, while an actual mammogram is depicted below.


What causes dense breast tissue?

It’s not clear why some women have a lot of dense breast tissue and others do not. You may be more likely to have dense breasts if you:

  • Are younger — Breast tissue tends to become less dense as you age, though some women may have dense breast tissue at any age.
  • Have a lower body mass index — Women who have less body fat are more likely to have denser breast tissue compared to women who have more body fat.
  • Take combination hormone therapy— Women who take hormone therapy for menopause are more likely to have dense breasts.

Are other tests more effective at finding cancer in women who have dense breasts?

They can be. Those tests include:

  • 3D mammogram (breast tomosynthesis). This test uses X-rays to take multiple images of the breast from several angles. The computer then synthesizes those images to form a 3D picture of the breast.
  • Molecular breast imaging (MBI). MBI uses a special kind of camera called a gamma camera and a radioactive tracer. The tracer is injected into a vein in your arm and MBI analyzes the way tissues react to the tracer. Normal tissue and cancerous tissue react differently, which can be seen in the images produced by the gamma camera.
  • Magnetic resonance imaging (MRI). MRI uses magnets instead of radiation to create images of the breast. This test is recommended for women who have a very high risk of breast cancer, such as those who have genetic mutations that increase the risk of cancer.

Every test has pros and cons. Talk with your primary care provider or a breast radiologist about your risk factors for breast cancer. Together you can decide whether additional screening tests are right for you.

A version of this article originally appeared on Mayo Clinic Health System blog.

Richard Ellis, M.D.

Dr. Ellis is a radiologist at the Mayo Clinic Health Systems branch in La Crosse, Wisconsin. He specializes in comprehensive breast diagnostic evaluation and intervention.

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