DEAR MAYO CLINIC: I just had a mammogram, and I was told that I have dense breast tissue. What does that mean?

ANSWER: About half of women are considered to have dense breasts, based on the appearance of their breast tissue on a mammogram. The main concerns about dense breast tissue are that it makes breast cancer screening more complex and increases the risk of breast cancer. Here are some answers to questions you might have:

What is dense breast tissue?

Breast tissue is composed of milk glands, milk ducts and supportive tissue, which make up dense tissue. Breasts also include fatty tissue, which is nondense tissue. When viewed on a mammogram, dense breasts have more dense tissue than fatty tissue.

Nondense breast tissue typically appears dark and transparent on a mammogram. In contrast, dense breast tissue appears as a solid white area, 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 to dense tissue and assigns a level of breast density. These levels are determined by results from the Breast Imaging Reporting and Data System from the American College of Radiology.

The levels of density are designated by letters:

What causes dense breast tissue?

It’s not clear why some women have a lot of dense breast tissue and others don’t. Dense breasts are more likely if you:

Why does breast density matter?

Having dense breast tissue won’t affect your daily life. However, it increases the chance that breast cancer may go undetected by a mammogram, since dense breast tissue can mask a potential cancer. It also increases your risk of breast cancer, though healthcare professionals aren’t yet certain why.

Do breast cancer screening recommendations change for dense breasts?

Women with dense breasts but no other risk factors for breast cancer are considered to have a higher risk of breast cancer than average. Dense breast tissue makes it more challenging to interpret a mammogram, since cancer and dense breast tissue both appear white on a mammogram. This may increase the risk that cancer won’t be detected on a mammogram.

However, mammograms are still effective screening tools. The most common type is a digital mammogram, which is more effective at detecting cancer. It saves images of your breasts as digital files, allowing for more detailed analysis.

Are other tests more effective?

Other tests carry both risks and benefits, although MRI and molecular breast imaging have demonstrated superior cancer detection in women with dense breasts.

Supplemental tests for breast cancer screening can include:

Every test has pros and cons. Talk with your healthcare professional about your breast cancer risk factors. Together, you can decide whether additional screening tests are right for you.

—Cameron Leitch, M.D., Radiology, Mayo Clinic Health System, Eau Claire, Wisconsin

(Mayo Clinic Q & A is an educational resource and doesn’t replace regular medical care. This Mayo Clinic Q&A represents inquiries this healthcare expert has received from patients. For more information, visit www.mayoclinic.org.)

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