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Dense breast tissue: What it means to have dense breasts

If a recent mammogram showed you have dense breast tissue, you may wonder what this means for your breast cancer risk. Doctors know dense breast tissue makes breast cancer screening more difficult and it may increase the risk of breast cancer.

In the United States, laws require doctors in some states to inform women when mammograms show they have dense breasts. But just what women should do in response isn't clear.

What is dense breast tissue?

Dense breast tissue refers to the appearance of breast tissue on a mammogram. It's a normal and common finding.

Breast tissue is composed of milk glands, milk ducts and supportive tissue (dense breast tissue), and fatty tissue (nondense breast tissue). When viewed on a mammogram, women with dense breasts have more dense tissue than fatty tissue.

On a mammogram, nondense breast tissue appears dark and transparent. Dense breast tissue appears as a solid white area on a mammogram, which makes it difficult to see through.

How do doctors determine if you have dense breast tissue?

The radiologist who analyzes your mammogram determines the ratio of nondense tissue to dense tissue and assigns a level of breast density.

Levels of density are described using results reporting system called BI-RADS. The levels of density are:

  • Almost entirely fatty indicates that the breasts are almost entirely composed of fat. About 1 in 10 women have this result.
  • Scattered areas of fibroglandular density indicates there are some scattered areas of density, but the majority of the breast tissue is nondense. About 4 in 10 women have this result.
  • Heterogeneously dense indicates that there are some areas of nondense tissue, but that the majority of the breast tissue is dense. About 4 in 10 women have this result.
  • Extremely dense indicates that nearly all of the breast tissue is dense. About 1 in 10 women have this result.

In general, a woman whose breasts are classified as heterogeneously dense or extremely dense is considered to have dense breasts. About half of women undergoing mammogram testing have dense breasts.

Breast density — The four levels

Breast tissue is composed of milk glands, milk ducts and supportive tissue (dense breast tissue) and fatty tissue (nondense breast tissue). Radiologists use mammogram images to grade breast tissue based on the proportion of dense to nondense tissue. According to the BI-RADS reporting system, the levels are (from left to right) almost entirely fatty, scattered fibroglandular density, heterogeneously dense and extremely dense.

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. Women in their 40s and 50s are most likely to have dense breast tissue. Your breast tissue tends to become less dense as you age, though some women may have dense breast tissue at any age.
  • Are premenopausal. Premenopausal women are more likely to have dense breasts.
  • Take hormone therapy for menopause. Women who take combination hormone therapy to relieve signs and symptoms of menopause are more likely to have dense breasts.

Why does breast density matter?

Having dense breasts affects you in two ways:

  • Increases the chance that breast cancer may go undetected by a mammogram, since dense breast tissue can mask a potential cancer
  • Increases your risk of breast cancer, though doctors aren't certain why

How does dense breast tissue affect breast cancer screening?

Dense breast tissue makes it more difficult to interpret a mammogram, since cancer and dense breast tissue both appear white on a mammogram. Very dense breasts may increase the risk that cancer won't be detected on a mammogram.

Women with very dense breasts who develop breast cancer are more likely than those with very fatty breasts to have their cancer discovered after a recent normal mammogram.

Mammograms are recommended for breast cancer screening in women with dense breast tissue.

Women with dense breasts, but no other risk factors for breast cancer, are considered to have an average risk. Most medical organizations recommend women with an average risk of breast cancer begin annual mammogram testing at age 40, though not all agree.

Undergoing mammogram screening every year may help detect a breast abnormality sooner in women with dense breast tissue, compared with undergoing mammogram screening every two years or less frequently.

Despite concerns about detecting cancer in dense breasts, mammograms are still effective screening tools. Digital mammograms, which save images of your breasts as digital files instead of film, allow for more detailed analysis and are more effective at finding cancer in dense breast tissue. Most mammogram machines in the United States are digital.

Are other tests more effective?

There's some evidence that additional tests may make it more likely that breast cancer is detected in dense breast tissue. But additional tests carry additional risks, and no additional testing method is proved to reduce the risk of dying of breast cancer.

You and your doctor may consider additional or supplemental testing based on your other risk factors and your personal preferences.

Supplemental tests for breast cancer screening may include:

  • 3-D mammogram (breast tomosynthesis). Tomosynthesis uses X-rays to collect multiple images of the breast from several angles. The images are synthesized by a computer to form a 3-D image of the breast.
  • Breast MRI. MRI uses magnets to create images of the breast. MRI doesn't use radiation. Breast MRI is recommended for women with a very high risk of breast cancer, such as those with genetic mutations that increase the risk of cancer.
  • Breast ultrasound. Ultrasound uses sound waves to analyze tissue. A diagnostic ultrasound is commonly used to investigate areas of concern discovered on a mammogram. This is a different type of exam from a screening breast ultrasound.
  • Molecular breast imaging (MBI). MBI uses a special camera (gamma camera) that records the activity of a radioactive tracer. The tracer is injected into a vein in your arm. Normal tissue and cancerous tissue react differently to the tracer, which can be seen in the images produced by the gamma camera.

It's not clear how often to undergo these additional tests.

Every test has pros and cons. While each test is proved to find more breast cancers than a mammogram, none of these tests is proved to reduce the risk of dying of breast cancer.

Supplemental screening test Pros Cons
3-D mammogram
  • Estimated to detect an additional 1-2 cancers per 1,000 women
  • Done at the same time as a standard mammogram
  • Reduces the need for additional testing for areas of concern that aren't cancer
  • Exposes you to additional radiation, though levels are still very low
  • Not available everywhere, though becoming more common
Breast MRI
  • Estimated to detect 18 or more additional cancers per 1,000 women
  • No additional radiation exposure
  • Widely available
  • Likely to find areas of concern that aren't cancer, but that require additional imaging or a biopsy
  • Requires injection of contrast material
  • Expensive test not often covered by insurance unless you have a very high risk of cancer
Breast ultrasound
  • Estimated to detect an additional 3-4 cancers per 1,000 women
  • No additional radiation exposure
  • Widely available
  • Likely to find areas of concern that aren't cancer, but that require additional imaging or a biopsy
  • Quality of exam dependent on experience of person doing the test
Molecular breast imaging
  • Estimated to detect an additional 7 cancers per 1,000 women
  • Less likely to find areas of concern that aren't cancer
  • Involves injection of a radioactive tracer, which exposes you to a very low level of radiation
  • Available at few medical centers

Last updated: February 26th, 2015

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