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What is Breast Density and Why Does It Matter?

By Jennie Dale

Roughly 40% of Canadian women, meaning about 3 million women, have what is known as “dense breasts.” Dense breasts are normal and common, but they also pose cancer risks and screening challenges. Breast density can have a significant impact on cancer detection and the treatment and prognosis of a diagnosed cancer. Many women in Canada are unaware of their breast density, impacting their screening and their ability to be their own breast health advocate. Why is knowing and understanding your breast density so important?

What are dense breasts? 

  • Dense breasts are simply breasts that have more glandular tissue than fat
  • Lumpy breasts are not the same as dense breasts.   You can’t tell density by size or touch.
  • Your density is assessed by the radiologist reading your mammogram.
  • Radiologists use one of four categories to classify your density. In provinces where words are used, dense breasts are described as: “heterogeneously dense breasts” and “extremely dense breasts.” In provinces where percentages are used, having more than 50% dense tissue means you have dense breasts. 

Dense breasts verus fatty breasts

Four reasons why you want to know if you have dense breasts:

1. The accuracy of a mammogram is impacted by dense breast tissue:

Dense breasts are the number one reason for cancer being missed by mammography. In women with fatty breasts, mammograms are 98% accurate, but in women with dense breasts, mammograms may be less than 50% accurate[i] because of a masking effect: both cancer and dense tissue show up white on a mammogram image. Radiologists compare it to finding a snowball in a snowstorm. As a result, there can be a delay in a cancer diagnosis and a worse prognosis for women with dense breast tissue. 

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

Dense breasts are an independent risk factor for breast cancer. Cancer is 4-6 times more likely in women with the highest level of density. [ii] Most women and family doctors do not know this, but dense breasts are an even greater risk factor than having a family history of breast cancer.[iii]

3. Women with dense breasts have a higher risk of cancer in the other breast:

Women who’ve been diagnosed with breast cancer and have dense breasts are nearly twice as likely to develop cancer in the opposite breast.[iv]

4. Women with dense breasts have a higher risk of an interval cancer:

Women with dense breasts are 18 times more likely to present with an interval cancer [v] (a cancer that becomes ‘feelable’ after a normal mammogram).  Interval cancers are larger, rapidly growing and have a much worse prognosis than mammogram-detected cancers.

How you can find out your breast density:

  • You are entitled to all information on file about your health, but accessing the information depends on where you live.  
  • In Ontario, Prince Edward Island, and New Brunswick, you can just ask your doctor, “What did my mammogram report say about my density?”  In other provinces, like British Columbia and Manitoba, the information is not reported to the family doctor so women have to submit a request for health information form to their screening program.

What should you do if you have dense breasts? 

  • If you have dense breasts, a mammogram is not enough. Many women with dense breasts have a normal mammogram and then later find a lump that had been missed, often resulting in a later-stage, more aggressive cancer.  Speak with your doctor about your density and any additional risk factors, but be aware that some doctors are not informed about the risks of dense breasts. If you learn you have dense breasts, you can consider supplementary screening, such as ultrasound and be more vigilant about self-check.
  • A growing body of research demonstrates that additional screening, such as ultrasound, can increase detection of early stage cancer: ultrasound finds an additional 3-4 cancers per 1000 women with dense breasts[vi]. Ultrasound can find cancer when it is small and node negative, reducing the need for aggressive treatment and improving rates of survival. [vii]
  • Early detection leads to less invasive treatment and improves outcomes.

Until there’s a cure, find it small. 

Dense Breasts Canada is seeking to raise awareness of the risks and screening challenges of dense breasts. Please help us spread the word. We are also looking for women with a missed or delayed cancer diagnosis due to dense breast tissue to share their stories.  Please contact us:   info@densebreastscanada.ca

Website:     www.densebreastscanada.ca 
Facebook:  www.facebook.com/densebreastscanada/


[i]   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. http://www.surgery.usc.edu/uppergi-general/downloads/readinglist/2002-10-comparison-of-performance-screening-mammography.pdf

[ii] 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.  http://www.nejm.org/doi/full/10.1056/NEJMoa062790#t=article

[iii] 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.  http://jamanetwork.com/journals/jamaoncology/article-abstract/2599991

[iv]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. https://www.ncbi.nlm.nih.gov/pubmed/28135395

[v] 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 https://breast-cancer-research.biomedcentral.com/articles/10.1186/bcr2942

[vi] 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.  http://www.ncbi.nlm.nih.gov/pubmed/18477782

[vii] Saadatmand, S.,Bretveld,R., Siesling,S., Tilanus-Linthorst,M. (2015, Oct. 6). Influence of tunor stage at breast cancer detection on survival in modern times:population bases study in 173787 patients. The BMJ. http://www.bmj.com/content/351/bmj.h4901