Breast density

Special thanks to Dense Breasts Canada for providing this section.

26,000 Canadian women are diagnosed annually with breast cancer, but not all cancers are found early. Mammography has reduced mortality by 40%, but is limited in women with dense breasts. What can be done to find cancer earlier in these women? Dense Breasts Canada discusses the importance of knowing and understanding your breast density.

What is breast density?

A woman’s breast is composed of three kinds of tissue: fibrous, glandular, and fatty.

  • Fibrous tissue provides structure, supporting the rest of the breast tissue.
  • Glandular tissue makes up the parts of the breast that produce milk (the lobes) and the tubes that carry the milk to the nipple (the ducts).
  • Fatty tissue makes up the rest of the breast, giving it its size and shape.

Breast density is the proportion of fibrous and glandular tissue to the amount of fatty tissue. Denser breast have more fibrous and glandular tissue, and less dense breast have more fatty tissue.

Breast density has nothing to do with how much the breast weighs. It’s about how the breast appears on a mammogram. Fibrous and glandular tissue is called dense tissue because, on a mammogram, it makes it harder to see unusual things that could be cancer.

The 4 categories of breast density

There are four categories of breast density, from 0% dense tissue to 100%. Breasts that have more than 50% dense tissue (Category C and D) are considered dense breasts.

Category Percentage of dense tissue Description Percentage of women who are in this category
A Less than 25% Almost entirely fatty About 10%
B Between 25 and 50% Scattered areas of density About 40%
C Between 51 and 75% Heterogeneously dense About 40%
D More than 75% Extremely dense About 10%

Side-by-side mammograms of four different breasts, illustrating each of the categories of breast density.

How common are dense breasts?

  • Dense breasts are normal and common. 43% of women over age 40 have dense breasts (Sprague et al., 2014).
  • Having dense breasts does not mean you will develop cancer.

How is breast density determined?

  • Breast density is determined by the radiologist when viewing a mammogram. Some provinces are beginning to use software to assess breast density.
  • Breast density cannot be determined by breast size or touch. Lumpy breasts are not the same as dense breasts.  Both fatty and dense breasts can feel soft, firm or lumpy.

Five reasons why breast density matters

1. Mammogram accuracy can be impacted by dense breast tissue

  • Dense breasts are the number one reason for cancer being missed by mammography. A mammogram can be assessed as “normal” or “negative,” even when there is a cancer.
  • Cancer and dense breast tissue both show up white on a mammogram, creating a camouflage effect and making it hard for radiologists to detect cancer.
  • In women with the highest category of density (over 75% dense tissue), mammograms are only 50% accurate, meaning 1 in 2 cancers will be missed (Kolb et al., 2012).

Cancer in a fatty breast is easy to spot with a mammogram, because there is good contrast between the cancer and the normal tissue. It is hard to see cancer in a mammogram of a dense breast, because dense breast tissue and cancer both appear as white.

2. Women with dense breasts have an increased risk of breast cancer

  • Dense breasts are an independent risk factor for breast cancer.
  • The higher the density, the higher the risk of cancer.
  • Cancer is 4-6 times more likely in women in the highest category of density than in the fatty category (Boyd et al., 2007).

3. Dense breasts pose a greater risk than family history

  • Recent research findings show that dense breasts are an even greater risk factor than having a family history of breast cancer (Engmann et al., 2017).

4. Higher risk of an interval cancer

  • Women with the highest category of density are 18 times more likely to have an interval cancer – a cancer that becomes “feelable” after a normal mammogram result and is detected in between screenings (Boyd et al., 2007).
  • Interval cancers are larger, more aggressive and have a worse prognosis than mammogram-detected cancers. Tumour stage at diagnosis influences overall survival rate and therefore diagnosis of breast cancer at an early stage is critical. (Saadatman et al., 2015)

5. Higher risk of cancer in the other breast

  • Women who have been diagnosed with breast cancer and have dense breasts are nearly twice as likely to develop cancer in the opposite breast as women with non-dense breasts (Raghavendra et al., 2017).

Factors that can affect breast density

  • Age: Women under age 50 have the highest degree of density.
  • Menopause: Density usually decreases after menopause, but not always.
  • Ethnicity: Women of Asian descent usually have a higher level of density.
  • Heredity: Breast density can be inherited.
  • Hormone Therapy: Density increases in women on hormones (usually prescribed to manage menopausal symptoms).
  • Aromatase Inhibitors and Tamoxifen: Prescribed for breast cancer. Decreases density.

(Hashemi et al., 2017)

How can I find out my breast density?

  • Women need to take steps on their own to find out their density as they are not routinely informed. In some provinces, this information is not even relayed to the GP.  The way to find out one’s breast density varies by province.
  • In ON, AB, NB, QC, and PEI, women can ask the family doctor, “What does my mammogram report say about my density?” In most cases, the information is in the report.
  •  In BC, MB, and NS, this information is not shared with the family doctor. Women must submit a request for their records to the screening program.
  • In SK, NL, MB, YT, and NWT, even when a request for records is submitted, only women in Category D (over 75% dense tissue) can find out. There is no information recorded for women in Category C.
  • Further details on how to find out your density are found at

What should I do if I have dense breasts?

  • Speak with your doctor about your density and the associated risks, any additional risk factors, and the best screening options for you.
  • Perform regular self-exams between screenings.
  • To improve early cancer detection in dense breasts, consider additional screening, such as ultrasound and/or MRI.
  • Consider the modification of lifestyle factors such as diet, moderate exercise, decreased alcohol intake and decreased hormone use to decrease cancer risk.
  • Continue having mammograms because they can detect calcifications, which can be the earliest sign of cancer.
  • If you are diagnosed with breast cancer, consider having a breast MRI to ensure no additional tumours are hidden before deciding on lumpectomy vs. mastectomy.

Research suggests supplemental screening

  • Supplemental screening, such as ultrasound, can increase detection of early stage cancer obscured by dense breast tissue.
  • Ultrasound finds an addition al 3-4 cancers per 1000 women screened. These are cancers not detected by mammogram. Ultrasound finds cancers small and node negative, reducing the need for aggressive treatment and improving rates of survival (Berg et al., 2008).
  • Early detection of cancer is critical for less invasive treatment and improved outcomes.

Until there’s a cure, find it small.

About Dense Breasts Canada

Dense Breasts Canada is a nonprofit organization made up of breast cancer survivors and healthcare professionals dedicated to informing women of the risks associated with dense breast tissue.

More resources

The U.S. Centers for Disease Control and Prevention (CDC) has excellent information on dense breasts.

DenseBreast-info also has more resources about breast density for both patients and physicians.


Berg, W., Blume, J., Cormack, J. et al. (2008). Combined screening with ultrasound and mammography vs. mammography alone in women at elevated risk of breast cancer. JAMA, 299: 2151-63.

Boyd, N., Guo, H., Martin, L., Sun, L., Stone, J., Fishel, E., Jong, R., Hislop, G., Chiarelli, A., Minkin, S., and Yaffe, M.  (2007, Jan.). Mammographic density and the risk and detection of breast cancer. New England Journal of Medicine, 356:227-236.

Boyd, N., Martin, L.,  Yaffe, M and Minkin, S. (2011).Mammographic density and breast cancer risk: Current understanding and future prospects Breast Cancer Res 13:223

Engmann N., Golmakani M, Miglioretti D, Sprague, B., Kerlikowske, K., for the Breast Cancer Surveillance Consortium. (2017, Feb 2.) Population-Attributable Risk Proportion of Clinical Risk Factors for Breast Cancer. JAMA Oncol.

Hashemi, E., Haghighat, S., Olfatbakhsh, A., Harunda,T. (2017). Investigating the Factors Affecting the Mammographic Density of Breast Tissue in Patients Referred to the Breast Cancer Research Center, Iran. Multidisciplinary Cancer Investigation.

Kolb, T., Lichy, J., & Newhouse, J. (2002). Comparison of the performance of screening mammography, physical examination, and breast US and evaluation of factors that influence them: an analysis of 27,825 patient evaluations. Radiology. (225):165–17.

Raghavendra, A., Sinha, A. K., Le-Petross, H. T., Garg, N., Hsu, L., Patangan, M., Bevers, T. B., Shen, Y., Banu, A., Tripathy, D., Bedrosian, I. and Barcenas, C. H. (2017). Mammographic breast density is associated with the development of contralateral breast cancer. Cancer, 123: 1935–1940.

Saadatmand, S.,Bretveld,R., Siesling,S., Tilanus-Linthorst,M. (2015, Oct. 6). Influence of tumor stage at breast cancer detection on survival in modern times:population based study in 173787 patients. The BMJ.

Sprague, B., Gangnon, R., Burt, V., Trentham-Dietz, A., Hampton, J., Wellman, R., Miglioretti, D. (2014). Prevalence of Mammographically Dense Breasts in the United States. JNCI Journal of the National Cancer Institute, 106 (10),