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Breast Density Information for Ontario Breast Screening Program Participants

The following is information about breast density, how you can find out your breast density, how breast density can affect your health and recommendations for screening (checking) people with dense breasts.

About Breast Density

  • Breasts have 2 kinds of tissue:
    • Fatty tissue
    • Fibroglandular tissue, which is made up of ducts, glands for making milk and supportive tissue
  • A breast is considered dense when it has a lot of fibroglandular tissue, which looks white on a mammogram (the fatty tissue looks grey or black). Having some dense breast tissue is normal and very common.
  • A mammogram is an X-ray image of the breast that checks for breast cancer. High breast density can make finding breast cancer hard because cancer and dense breast tissue both look white on a mammogram. It is important to have regular mammograms if you are eligible to get them and to know what is normal for you.
  • Women with a lot of dense breast tissue have a higher chance of getting breast cancer, but it does not mean they will get breast cancer for sure (see Breast Density and Chance of Getting Breast Cancer).

Reporting Breast Density

  • Breast density is determined when you get your mammogram. The reading radiologist (a doctor who specializes in medical imaging) figures out your breast density using your mammogram. As of July 2021, the reading radiologist gives 2 breast density measures on your Ontario Breast Screening Program screening mammogram report:
    • Percent mammographic density, which is a visual estimate of how much dense breast tissue is on your mammogram. This measure is reported as either less than 75%, or 75% or higher.
    • A BI-RADS breast density category, which is a visually estimated description of the volume of dense breast tissue on your mammogram. BI-RADS stands for Breast Imaging Reporting and Data System. This measure has 4 possible categories reported as a letter, from A to D. “BI-RADS A” has the least amount of dense tissue and “BI-RADS D” has the most amount of dense tissue.
      • Category A – the breasts are almost all fatty (not dense) tissue
      • Category B – the breasts mainly have non-dense tissue with some scattered areas of fibroglandular (dense) tissue
      • Category C – the breasts are heterogeneously dense (a mixture of fatty and dense tissue)
      • Category D – the breasts are extremely dense (almost entirely dense breast tissue)

Eight side-by-side images show the appearance of breasts at each of the 4 categories of density, as described in the main text.

Breast/mammographic images on pages 128-130 of the ACR BI-RADS® Atlas, 5th edition
(https://www.acr.org/-/media/ACR/Files/RADS/BIRADS/Mammography-Reporting.pdf)

How Aging, Menopause and Hormone Replacement Therapy Affect Breast Density

  • Breast density is not constant throughout someone’s life.
  • Breasts change with age. Younger women have denser breasts than older women. This difference is normal because younger women have higher levels of the female hormone estrogen. As women stop having periods and reach menopause, estrogen levels drop and their breasts often become more fatty and less dense.
  • Taking hormone replacement therapy (HRT) for menopause can make breasts denser. Talk to your family doctor or nurse practitioner if you have any questions about your HRT medication.

Finding Out Your Breast Density Information From Your Mammogram

  • Breast density can only be determined when you have a mammogram. Breast density cannot be measured through a physical exam and it is not related to breast size or firmness.
  • A family doctor or nurse practitioner will get a copy of their patient’s Ontario Breast Screening Program (OBSP) screening mammogram report from the site where the mammogram was performed. To find out your breast density, contact your family doctor or nurse practitioner for the breast density measure found in your OBSP screening mammogram report. If you do not have a family doctor or nurse practitioner, you can contact your site for a copy of your OBSP screening mammogram report to find out your breast density.
  • To find out your breast density from a mammogram that was done outside the OBSP, please contact the site where you had the mammogram and they may be able to give you information.

Breast Density and the Chance of Getting Breast Cancer

  • High breast density relates to breast cancer for 2 reasons:
    • Breast cancer and masses appear white on a mammogram. Dense tissue also appears white, which can make it harder to find breast cancer on a mammogram.
    • Research shows that the chance of getting breast cancer goes up with more dense breast tissue.
  • One study looked at the association between mammographic density and the chance of breast cancer. In groups of people who qualified for breast screening, including Canadian women ages 50 to 69, higher breast density was associated with a higher chance of getting breast cancer. Women with 75% or more dense tissue had 4.7 times the chance of developing breast cancer compared to women with little dense tissue (less than 10%).
  • Dense breasts are just one of many reasons someone’s chance of getting breast cancer could be higher.
    • To learn more about the things you can do to change your chance of getting breast cancer, such as reducing alcohol intake, please visit Risk factors you can change or control at mycanceriq.ca.
    • To learn more about things you cannot change that could increase your chance of getting breast cancer, such as a family history of breast cancer, please visit Risk factors you can’t change or control at mycanceriq.ca.
  • You cannot do much to change your breast density however it can be affected by other factors (see How Aging, Menopause and Hormone Replacement Therapy Affect Breast Density).
  • Some factors do not affect your breast density but may still affect your chance of getting breast cancer. For example, being overweight or obese does not affect breast density but being obese is a reason someone’s chance of getting breast cancer could be higher.
  • You can also talk to your family doctor or nurse practitioner about these factors and your overall chance of getting breast cancer.

Screening for People with Dense Breasts

  • If your mammogram shows that your breasts have 75% or more dense tissue, the radiologist will recommend that you come back in 1 year for a mammogram instead of in the usual 2 years.
  • If your breast density is or becomes less dense (less than 75% mammographic density) and the Ontario Breast Screening Program has no other reason for you to come back in 1 year, you will be invited back to get a screening mammogram in 2 years. One example of why you might get invited back in 1 year instead of 2 is if you have 2 or more first-degree female relatives (mother, sister, daughter) who had breast cancer at any age.

Screening Tests Other Than Mammography

  • Currently, there is not enough research to recommend other screening tests (e.g., magnetic resonance imaging or ultrasound) based only on breast density.