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Breast Density Information for Healthcare Providers

The following is information about breast density, how participants can access their breast density information, risk factors for getting breast cancer, and recommendations for screening people with dense breasts. The content on this page is for family doctors, nurse practitioners and Ontario Breast Screening Program sites.

Breast Composition

  • Breasts contain 2 types of tissue: fatty and fibroglandular tissue. Normal fibroglandular tissue appears dense (or white) on a mammogram, while fatty tissue appears non-dense (or black).

Reporting Breast Density

  • In the Ontario Breast Screening Program, breast density is determined from a mammogram and is interpreted by the reading radiologist. The radiologist reports 2 breast density measures:
    • Percent mammographic density, which is a visual estimate of the percentage of fibroglandular (dense) tissue in the breast. This measure is reported as either less than 75%, or 75% or higher.
    • A Breast Imaging Reporting and Data System (BI-RADS) breast density category, which is a visually estimated description of the volume of dense breast tissue on the mammogram. There are 4 categories: A, B, C and D (see Defining Breast Density).

Defining Breast Density

  • The Ontario Breast Screening Program defines high breast density as 75% or higher mammographic density. This definition is not based on Breast Imaging Reporting and Data System (BI-RADS) breast density categories. The program’s definition of high breast density is instead based on evidence from a systematic review of published research showing that women with 75% or higher dense tissue have a higher risk of developing breast cancer than women with less than 5% dense tissue.
  • A BI-RADS breast density category is a visually estimated description of the volume of dense breast tissue on the mammogram. The categories provide clinicians with awareness of the limitations of mammography because they help indicate whether masses may be obscured by dense tissue. The sensitivity of mammography for non-calcified lesions decreases as the BI-RADS breast density category increases. If the reading radiologist finds the breasts are not of equal density, the denser breast is used to categorize breast density.
  • There are four BI-RADS breast density categories. “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 entirely fatty
    • Category B – there are scattered areas of fibroglandular density
    • Category C – the breasts are heterogeneously dense, which may obscure small masses
    • Category D – the breasts are extremely dense, which lowers the sensitivity of mammography

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)

Breast Imaging Reporting and Data System (BI-RADS) Breast Density Categories, Less Than 75% Mammographic Density and the Ontario Breast Screening Program’s (OBSP) Screening Interval

  • The OBSP has received questions about how the “BI-RADS C” category compares with less than 75% mammographic density. The 2 breast density measures cannot be compared (see Reporting Breast Density). “BI-RADS C” indicates that breasts are a combination of fatty tissue and fibroglandular (dense) tissue. Less than 75% mammographic density indicates that breast composition is less than 75% fibroglandular tissue.
  • Based on evidence, the program recommends that participants with 75% or higher mammographic density (i.e., high breast density) be recalled for their next screening mammogram in 1 year. A participant with less than 75% mammographic density will be recalled for their next screening mammogram by the program in 2 years (if there are no other factors that would indicate a 1-year recall, such as having a documented pathology of a concerning benign lesion [high risk lesion]). Currently, BI-RADS breast density categories are not used by the program to determine when a participant should be recalled for their next screening mammogram.
  • Participants who are concerned about their breast density and breast cancer risk can talk with their family doctor or nurse practitioner to discuss whether they have other risk factors (e.g., family history) that may increase their risk of getting breast cancer. They can also discuss ways to reduce their breast cancer risk (see Information to Share with Participants About Breast Density and Modifiable Lifestyle Choices).

How Ontario Breast Screening Program (OBSP) Participants Can Find Out Their Breast Density

  • As of July 2021, all OBSP screening mammogram reports contain a participant’s breast density measures (i.e., whether they have 75% or higher mammographic density and their Breast Imaging Reporting and Data System [BI-RADS] breast density category). A family doctor or nurse practitioner will get a copy of their patient’s OBSP screening mammogram report from the OBSP site where the mammogram was performed.
  • People who get screened through the program can contact their family doctor or nurse practitioner to find out their breast density. Participants without a family doctor or nurse practitioner can contact their OBSP site for a copy of their OBSP screening mammogram report to find out their breast density.
  • Ontario Health (Cancer Care Ontario) sends result letters to participants to notify them of their mammogram result when the result is normal. We do not send a copy of the OBSP screening mammogram report to the participant. If a mammogram result is abnormal and further testing is needed, the OBSP site, family doctor, or nurse practitioner will contact the participant directly. Through result letters, we notify participants with high breast density (i.e., 75% or higher mammographic density) and normal screening results that they will be recalled for screening in 1 year instead of 2 years due to dense breast tissue on their mammogram.

High Breast Density and Developing Breast Cancer

  • Higher density relates to the risk of developing 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 risk of breast cancer increases as the amount of dense tissue in the breast increases.
  • High breast density is one of several factors (e.g., BRCA1/2 gene mutations, personal history of breast cancer, obesity, physical inactivity) that can affect someone’s risk of developing breast cancer. In a study of screen-eligible populations that included Canadian women ages 50 to 69 from Ontario and British Columbia, higher breast density was associated with a higher risk of developing breast cancer. Women with 75% or more dense tissue had 4.7 times the risk of developing breast cancer compared to women with little dense tissue (less than 10%).

Information to Share With Participants About Breast Density and Modifiable Lifestyle Choices

  • Participants should be aware that the presence of dense breast tissue is normal and very common, and it is important to get regular mammograms.
  • Participants cannot do much to change their breast density however it can be affected by other factors (see How Aging, Menopause and Hormone Replacement Therapy Affect Breast Density).
  • Participants can visit Breast Density Information for Ontario Breast Screening Program Participants at cancercareontario.ca/breastdensity for information on breast density and risk factors for breast cancer.
  • Although high breast density may increase the risk of developing breast cancer, modifiable lifestyle choices may also affect the risk of developing breast cancer. These modifiable lifestyle choices can affect breast cancer risk more than high breast density.
  • For information on modifiable breast cancer risk factors (e.g., alcohol consumption, adult weight gain, physical activity), please visit Risk factors you can change or control at mycanceriq.ca.
  • For information on non-modifiable breast cancer risk factors (e.g., genetics), please visit Risk factors you can’t change or control at mycanceriq.ca.

How Aging, Menopause and Hormone Replacement Therapy Affect Breast Density

  • Breast composition changes with age. Generally, 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 undergo menopause, estrogen levels decrease, and their breasts often become more fatty and less dense.
  • Taking hormone replacement therapy for menopause can increase breast density.
  • Some breast cancer risk factors do not affect breast density. For example, being overweight or obese does not affect breast density, but being obese is a risk factor for getting breast cancer.

Recommended Breast Screening Tests and Screening Interval Based on Breast Density

  • In the Ontario Breast Screening Program, participants with high breast density (75% or higher) and normal screening results are called back for screening in 1 year instead of 2 years.
  • If a participant’s breast density is or becomes less dense (less than 75% mammographic density) and there is no other reason for them to be recalled in 1 year (e.g., family history), they will be called back to get a screening mammogram in 2 years.
  • Supplemental screening is the provision of screening tests in addition to screening mammograms. The program does not recommend routine supplemental screening for people with dense breasts as part of an organized screening program because the benefit of supplemental screening is unknown. Examples of supplemental screening methods include ultrasound, magnetic resonance imaging (MRI) and digital breast tomosynthesis (DBT). The program’s 1-year recall recommendation is considered a shortening of the screening interval and not supplemental screening.
  • Currently, there is not enough scientific evidence to recommend supplemental screening (e.g., with ultrasound, MRI and DBT) based only on breast density. Supplemental screening for people with dense breasts may find breast cancer missed by a screening mammogram, but it may also increase the likelihood of a false-positive result, which would lead to unnecessary follow-up tests.
  • In Ontario, people with dense breasts who are concerned about their risk of developing breast cancer should talk with their family doctor or nurse practitioner. Individual decisions about supplemental breast screening should be made through a shared decision-making process involving a discussion between a family doctor or nurse practitioner and their patient.
  • The program will continue to monitor the evidence on supplemental screening and will update this guidance as more evidence becomes available.