Overview

Breasts come in all shapes and sizes. The tissue inside your breasts can be different types too. Some breast tissue is fatty. Other breast tissue is dense. "Dense" means it's made of thick, fibrous tissue and milk glands.

You can't tell how dense your breasts are by looking in the mirror or feeling them. The mammogram report sent to your doctor tells how dense your breasts are. It's written by the radiologist who reads your mammogram.

You can learn how dense your breasts are from your mammogram report. There are four categories of breast density:

• Level A: Almost all fatty tissue (1 out of 10 women)
• Level B: Scattered areas of dense tissue, but mostly fatty tissue (4 out of 10 women)
• Level C: Mixed dense and fatty tissue, also called heterogeneous (4 out of 10 women)
• Level D: Extremely dense tissue (1 out of 10 women)

All of these breast types are normal. You only have dense breasts if the report says that your breasts are category C or D.

Things that can affect your breast density include your family history (genetics), being pregnant, and using estrogen hormone therapy. Your age can also make a difference. Breast tissue in younger women tends to be denser than in older women who have been through menopause.

If you have questions about your breast density or other concerns, get a copy of your mammogram report. Then talk to your doctor about it.

Why is it important to know about your breast density?

The more dense a breast is, the harder it is to see cancer on a mammogram image. That's because dense tissue looks white onscreen, just like cancer does.

Breast cancer tends to grow in dense breast tissue more often than in fatty breast tissue. So having dense breasts may slightly increase your risk for breast cancer.

Breast density is a risk factor for cancer. Your overall risk is based on facts like how old you are, whether you've ever had breast cancer before, and whether any of your close relatives, such as your mother or sister, have had breast cancer.

How are dense breasts screened for breast cancer?

If you have dense breasts but no other risk factors for breast cancer, a mammogram is the recommended test. There isn't enough evidence from studies to show that having other tests will help you.^{footnote 1}

If you have dense breasts and also have other risk factors for breast cancer, talk with your doctor to decide about screening.

• Mammogram only. A mammogram is recommended for all women who need breast cancer screening. A mammogram is the only test that can show tiny bits of calcium that can be a sign of cancer, even in dense breast tissue.
• Mammogram and breast ultrasound. These tests work well together for screening dense breast tissue. An ultrasound can show if a lump seen on a mammogram is a harmless fluid-filled cyst or something solid that could be a problem.
• Mammogram and MRI. MRI shows clear detail of breast tissue. But it often can't tell you what is cancer and what isn't. 

If breast cancer screening tests can't tell you that a spot is harmless, your next step is to decide whether to have a biopsy tested for cancer cells.

Your doctor or breast cancer screening centre may offer you other tests, such as digital breast tomosynthesis (also called 3D mammograms) or an abbreviated breast MRI (sometimes called a "fast MRI"). Talk with your doctor about whether these tests are right for you.