What is breast density?

Breast density is a description of the composition of breast tissue on a mammogram. All breasts are composed of fibrous and glandular tissue and fatty tissue. On a mammogram the fibrous and glandular tissue appears white and the fatty tissue appears black. A dense breast contains mostly white fibrous and glandular tissue with little fat.

Radiologists classify breast density on mammograms into four categories: almost entirely fatty, scattered areas of fibroglandular density, heterogeneously dense, and extremely dense. Women whose mammograms show heterogeneously dense or extremely dense tissue are considered to have “dense breasts”.

Image used by permission from the American College of Radiology

What is the science of breast density?

Dense breasts are normal and common. Half of all women have dense breasts. Breast density may decrease with age, but for many women it remains the same.

Breast density in the United States:

  • 10% of women have almost entirely fatty breasts
  • 40% of women have scattered fibroglandular tissue density
  • 40% of women have heterogeneously dense breasts
  • 10% of women have extremely dense breasts

Many studies demonstrate at least a moderate association of mammographic density and breast cancer risk. While the 10% of women with extremely dense breast have a 2 fold increased risk of developing breast cancer when compared with women who have low breast density, the 40% of women with heterogeneously dense breast tissue have only 1.2 fold increased risk. This makes density at least a moderate risk factor for breast cancer that is more important than traditional risk factors such as early menarche, late menopause, and late parity. Women with dense tissue have cancers that are larger, more likely lymph node positive, and of higher stage than women without dense tissue. It is important to note that breast density as the sole risk factor does not put women into a lifetime or 10-year high risk of breast cancer.

What will breast density notification look like?

Effective January 1st, 2019, Washington State law requires that mammography service providers inform patients if they have “dense breast tissue” on screening mammography. In addition to their screening mammography results, patients with dense breast tissue will also receive a letter containing the following statement:

“Your mammogram indicates that you may have dense breast tissue. Roughly half of all women have dense breast tissue which is normal. Dense breast tissue may make it more difficult to evaluate your mammogram. We are sharing this information with you and your health care provider to help raise your awareness of breast density. We encourage you to talk with your health care provider about this and other breast cancer risk factors. Together, you can decide which screening options are right for you.”

What does breast density mean for me?

Some patients who receive this notification letter will have questions and concerns about their breast cancer risk and what course of action they should take. The following key discussion points should be helpful:

  1. Women with dense breast should continue to get routine yearly screening mammograms.
  2. Having dense breast tissue alone does not place a patient at high risk for breast cancer.
  3. If the patient asks for specific information about their risk, start by reading the screening mammography report. The report should indicate if the breast tissue is either “Heterogeneously dense” or “Extremely dense.” Heterogeneously dense (40% of women) is associated with minimal risk (RR=1.2 compared to average breast density). Extremely dense (10% of women) doubles the risk of breast cancer compared to average density, similar to the risk associated with a 1st-degree family history (mother, sister, daughter).
  4. If your patient is interested in pursuing additional screening options, the next appropriate step is to determine if she would benefit from a breast cancer risk assessment. Does she have a first degree relative (mother, sister, daughter) who had premenopausal breast or ovarian cancer, or a male relative with breast cancer? Does she have a history of atypia (ADH, ALH) or LCIS on a previous breast biopsy? If your patient answers yes to any of the preceding questions, she would likely benefit from a risk assessment. This could be performed by a provider with experience in breast cancer risk model selection and interpretation, or by a genetic counselor in a cancer risk assessment program. Some breast centers provide lifetime risk information in their screening mammography reports.

What are the supplemental testing options?

There is no other recommended test to replace the screening mammogram. There are certain manifestations of cancer (for example, calcifications) that are only seen on mammography. Many centers now offer digital breast tomosynthesis (DBT), also known as 3D mammography. Many studies of DBT demonstrate increased cancer detection and decreased false positives compared to traditional (2D) screening mammography. The other “screening options” referred to in the letter are in addition to, and not instead of, a routine screening mammogram.

If, through history and risk assessment calculations, your patient has a high lifetime risk for breast cancer, she should be screened annually with both breast MRI and mammography. If a woman is being screened annually with MRI and mammography, no additional screening tests (such as ultrasound) are needed.

If your patient is at high risk but is unable to undergo screening breast MRI due to claustrophobia, pacemaker, contrast allergy, limited insurance coverage plan, or other reasons, she could consider screening breast ultrasound, the second best supplemental screening alternative. However, this screening method is not widely available, as many health centers have chosen not to offer it, in part because ultrasound depicts many fewer mammographically invisible cancers than does screening MRI.

What is this all going to cost?

Screening mammography, either with 2D screening or with Digital Breast Tomosynthesis (3D mammography) is a preventative service that does not incur out of pocket costs in our state. However, at most medical centers, additional/supplemental screening ultrasound or MRI tests incur an out-of-pocket expense for the patient, unless they have been assessed to have high risk. Assist the patient in making the best personal choice to meet her needs based on these factors, using a shared decision making process. All screening tests may find things that are not cancer, at times leading to additional imaging or biopsy and additional costs. However, the goal of screening is to detect cancer early when it can be treated more quickly, with fewer drugs and costs, and provide the best chance for cure.


1. Berg, WA., Harvey, JA. “Breast Density and Supplemental Screening.” [White paper] https://www.sbi-online.org/RESOURCES/WhitePapers/TabId/595/ArtMID/1617/ArticleID/596/Breast-Density-and-Supplemental-Screening.aspx. Accessed 8/27/2018.

2. American College of Radiology, “Breast Density Brochure.” https://www.acr.org/-/media/ACR/Files/Breast-Imaging-Resources/Breast-Density-bro_ACR_SBI.pdf. Accessed 8/27/2018.

3. “Breast Density, Breast Cancer Risk, and California Breast Density Notification Law SB 1538: Scenarios for Clinicians.” California Breast Density Information Group (CBDIG), March 2013; http://www.breastdensity.info/docs/DENSITY-SCENARIOS-FOR-CLINICIANS.pdf. Accessed 8/27/2018.


Content on this page was prepared by:

Robert L Gutierrez, MD, FSBI
Medical Director of Breast Imaging
Group Health/Kaiser Permanente Washington
Department of Radiology
Tacoma, WA

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