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What is all the talk about ‘breast density’?

Published on | Eric Brown


Michigan became the 21st state to enact breast density inform legislation. Governor Snyder signed Michigan’s breast density inform bill into law (SB 0879) and it becomes effective June, 2015.

The first density reporting law was enacted in Connecticut in 2009.  Another eight states are due to introduce density reporting legislation in 2015, according to Are You Dense Advocacy, Inc. The federal density reporting bill in the Senate and House is due for reintroduction in the new Congress soon.

So what is breast density?  Breast density essentially describes the ratio of breast:fat tissue, within the breast. Because of breast density, a mammogram my not find small lesions, which may be obscured by having significantly more breast tissue, than fat tissue within the breast (rendering the breast as being ‘dense’). According the the Journal of the National Cancer Institute (2014), the prevalence of dense breasts among US women of common breast cancer screening ages exceeds 25 million.

How is breast density reported?  Radiologists qualitatively rate breast tissue density from mammogram examinations using categories defined by the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS), similar to the reporting system used to define the significance of mammographic abnormalities.

The definition is as follows:

  • Almost entirely fat
  • Scattered fibroglandular densities
  • Heterogenously dense
  • Extremely dense


Legislation defines mammographically dense breasts as those rated “heterogeneously dense” or “extremely dense”.  The legislation will mandate that along with the obligatory letter women receive describing the result of their mammogram, including with this, will be a description of her breast density.  There most certainly will also be a commentary as to the significance of this result.

It is estimated that 43% of women aged 40-74 years have heterogeneously or extremely dense breasts, corresponding to approximately 27.6 million US women.  Furthermore, 45% of these women, are younger than age 50 years, further adding to the controversy of when screening mammogram should be performed.  If you add only a single additional test (ultrasound for example) to women who are felt to have dense breasts, the additional cost of their work up will be astronomical.

How do you address ‘dense’ breasts?  There are very limited data on what is the most effective way to address women with dense breasts.  Clinical trials suggest that the addition of screening ultrasound to mammography in this patient population provides an increase in cancer detection rates, but, also substantially increases the number of false positives (findings that turn out to be benign).  Other supplemental imaging modalities are also being evaluated, such as, breast MRI and breast tomosynthesis (3D mammography).  Obviously, the cost-effectiveness of supplemental screening for women with dense breasts has yet to undergo evaluation, and, certainly will complicate the issue.  So, there is a possibility that over 27 million women will be notified that their breasts are ‘dense’, but, will have no guideline or evidence based follow up/treatment plan.  This will put the primary care physicians, and, subsequently the specialists, in the position of determining ‘what is next’.

So, what does it all mean?  Breast density is very important when evaluating findings on a mammogram.  There is evidence that suggests a higher risk in this population of women, as well.  There is, however, reason to pause, as legislation requiring breast density reporting, heads towards a national level.  Breast specialists, are unsure of how to best address this issue, so, why are we in such a hurry to legislate?  It is not only a matter of cost-effectiveness, clinical significance, or perhaps the increase risk dense breast tissue might impart; what about a women’s level of stress with this knowledge?  Walking around with ‘dense breasts’ as defined in a relatively subjective manner, with at least some data suggesting a proportionate increase in the risk of one day developing breast cancer, would seem very unnerving, especially when we (breast specialists) are not exactly sure what to do about it.  Ultimately, this legislation will have a positive impact on women’s breast health.  The more we know, the earlier we can detect.  It remains to be seen what effect this legislation will have on how we screen, what modalities we add to the screening regimen of these women, and, how our insurance companies are going to address payment for this additional work up.

Be prepared, because it’s coming …

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