All About Breast Density

In my daily breast imaging practice, I hear this all the time – ”My breasts are so heavy and dense…” The unspoken question often is: “Am I at a higher risk of cancer?”

The answer is: “not necessarily.” How your breasts feel to you has nothing to do with “density.” So relax and let’s talk about “breast density.”

Breast density is a measure of how much “dense” tissue there is on the mammogram. It is something you find out only by looking at your mammogram.  Let me explain by first going over some basics of mammography.

Digital mammograms (and all x-ray images) are made up of pixels in shades of gray. Fat is dark gray, fibrous and glandular (we’ll call this combination “fibroglandular”) tissue is white. Different combinations of fat and fibroglandular tissue make up different shades of gray. The more fibroglandular tissue you have, the denser is your mammogram. If you have a lot of fat in your breasts, your mammograms are mostly gray. If you have a lot of fibroglandular tissue, your mammograms are mostly white. The majority of women are somewhere in between.

Radiologists roughly classify mammographic density into 4 categories: predominantly fat, scattered fibroglandular densities, heterogeneously dense, and extremely dense. When you get your mammography report, there will be a a statement describing your breast density as one of these four categories. You can have breasts that feel heavy but are actually fatty on mammograms, and you can have small breasts that are actually very dense on mammograms. You can’t tell the density by feeling, hefting or looking at the physical breast.

Why is it important to know your breast density?

To answer this, we need to have another piece of information: Cancers are white on a mammogram. So in a very dense breast, we radiologists are looking for white cats (cancers) in a snowy field (white breast tissue). Not always easy. Okay… so it’s not that dramatic, but you get the idea. It should come as no surprise therefore that the sensitivity (how many cancers are found) on mammography depends to a large extent on the breast density.
Mammography can pick up 90+% of cancers in a fatty breast (a small irregular white mass really stands out in a field of dark gray), but maybe only 50% of cancers in a very dense breast. I have personally scrutinized dense mammograms in women with cancers and, sometimes even knowing where the cancer must be, have not been able to “see” it on mammograms. This is called masking bias because the dense tissue “masks” the cancer.

Is masking bias the only reason people with dense breasts have been found to have a higher risk of cancer? Maybe not. Epidemiological research suggests that having dense breasts per se increases the risk of cancer, in addition to masking. Researchers are still working on figuring out why.

OK, now you understand mammographic density. But, oh no, my radiologist just told me I have dense breasts! What should I do? First, calm down. You’re in good company. The “dense” breast category includes women with heterogeneously and extremely dense breasts. Roughly broken down by categories, 10% of women are very fatty, 40% have scattered densities, 40% are heterogeneously dense, 10% have extremely dense breasts. So if you have dense breasts, you’re in a group that includes 50% of all women.

While for some women, as they go through menopause their breasts become fattier, for many of us, our breast density is what we have and it doesn’t change. There is not much we can do about it.

If you have a higher risk of breast cancer (family history, previous abnormal biopsies, history of radiation, etc) you might want to look into additional methods of screening, such as ultrasound or MRI, which together with mammography can significantly increase our ability to detect cancer. The recent advent of 3D mammography has really improved the sensitivity of mammography, so that’s helpful as well. But most importantly, TALK TO YOUR DOCTOR. Figure out a sensible way of screening for breast cancer. Don’t go nuts on your own.

OK. Now let’s circle back to the beginning.

Do heavy breasts mean dense breasts? NO.

Do lumpy breasts mean dense breasts? NO.

How do you know if you have dense breasts? Look at your mammogram report. Talk to your doctor if you do. Be sensible and don’t overreact.

RESOURCES:
1. http://ww5.komen.org/Breastcancer/Highbreastdensityonmammogram.html
2. http://www.diagnosticimaging.com/breast-imaging/breast-density-notification-laws-state-interactive-map
3. http://www.areyoudense.org/about/
4. http://download.springer.com/static/pdf/940/art%253A10.1186%252Fbcr2942.pdf?originUrl=http%3A%2F%2Fbreast-cancer-research.biomedcentral.com%2Farticle%2F10.1186%2Fbcr2942&token2=exp=1466086515~acl=%2Fstatic%2Fpdf%2F940%2Fart%25253A10.1186%25252Fbcr2942.pdf*~hmac=daf2832b7b940cdc05b34acaebc487e94da79d5b2ef4e0dce3756bd674b09bc1