Breast density is assessed by mammogram and is a measure of how much fat compared to other tissue make up a woman’s breasts. Breasts with a low proportion of fat are considered high density. Breasts with a high proportion of fat are considered low density.

The papers—one out of the National Cancer Institute (NCI) (study), the other out of Cancer Research and Biostatistics in Seattle (study)—found that breast density, when added to a group of other risk factors, helped improve the accuracy of predicting a woman’s overall risk of breast cancer. The Seattle group in particular found that a woman’s breast density alone was almost as important a predictor of risk as the very important risk factor, age.

That breast density is linked to breast cancer risk isn’t necessarily news, however. Many previous studies have found a strong link between women with higher density breasts and an increased risk of breast cancer. What is news is that these papers have done the complicated analyses and shown that breast density could be a valuable addition to tools that estimate women’s overall breast cancer risk. And it seems to predict risk in both pre and postmenopausal women, and as shown in the Seattle study, across different races and ethnicities.

More work is needed, though, before breast density is regularly used to assess breast cancer risk. The results from these papers need to be confirmed by other researchers, and beyond that, there need to be national standards for measuring breast density in women. Right now, the federal group that certifies mammography facilities doesn’t require that facilities determine the density of a women’s breasts when a mammogram is taken. And even if a facility does measure breast density, there currently aren’t any agreed-upon standards for how to classify the density of woman’s breast. One good example of this: the Seattle and NCI groups each used a slightly different measure of density in their studies. Only after issues like this are ironed out will women be able to get as accurate and comparable risk estimates as possible using breast density.

One intriguing side issue to the whole discussion is whether or not breast density is a modifiable risk factor—meaning, can women lower their breast density and thereby lower their risk of breast cancer. While there’s evidence that breast density can be changed by hormone use, tamoxifen use, or even some changes in diet, it’s unclear whether this directly and substantially affects a woman’s risk. It’s possible that the underlying factors that determine a woman’s natural breast density (some of which are genetic) are the things that really drive risk, so that short-term changes in breast density may only have little, if any, impact on risk. Only further studies can help shine light on this.

Regardless, breast density seems poised to become an important part of breast cancer risk assessment in the not too distant future. Although there’s a long way to go before science will be able to very accurately predict the breast cancer risk of an individual woman, every refinement in the practice empowers women with that much more information about their risk and the steps they can take to protect themselves from the disease.
> Next: Tips for Breast Health

—Written by Hank Dart
HCCP Staff

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