Measuring Breast Density

Have you ever gone for your annual screening mammogram and been told that you need additional imaging studies done because your breast tissue is “dense”? So what does this mean?

Breast density is the relative proportion of tissue to fat in a woman’s breast. In other words, the greater the quantity of breast tissue compared to the amount of fat, the denser the breast will be.

Dense tissue hinders the interpretation of a mammogram

On a mammogram, dense tissue looks white--and unfortunately so does that worrisome speck on your films that a breast-imaging radiologist is trying to find and evaluate. It’s sort of like trying to see a polar bear in a snow storm. It’s important to know because density can prevent early diagnosis of breast cancer for some.

So is there a polar bear actually wandering around in your mammogram films? Possibly not. But since breast density is now considered--just by itself--to be a sign of elevated breast-cancer risk, these pale spots must be found and diagnosed.

Breast density a risk predictor?

And it’s important for another reason to be able to accurately calculate the density of breast tissue: Radiologists think breast density might turn out to be a good predictor of a woman’s breast-cancer risk.

Two research studies presented recently in Philadelphia at the 52nd Annual meeting of the American Association of Physicists (!) in Medicine (AAPM) compared a number of technologies for measuring breast-tissue density.

One study compared the accuracy of 2 existing techniques (the cone-beam CT and the breast MRI) in quantifying breast density. Using each of the techniques, the researchers scanned 20 pairs of breasts and found that the 2 procedures furnished density values that were comparable. The second study suggested that a third technique--dual-energy mammography--might also be useful for measuring breast density. This last technology uses dual-energy x-ray imaging to isolate and then quantify the signals from 2 overlapping tissues within a breast; the procedure can do this because it can detect slight variations in the energy fields of the 2 tissues. (No wonder they had to get physicists involved!)

That lingering problem

A key problem with dense breasts, however, still needs to be addressed: how to consistently find and evaluate the polar bear in the snow storm. This is the reason why digital mammography is so important: It can find abnormalities in breast tissue (including dense tissue) that are missed on traditional analog film mammograms. 

So there are really 2 issues regarding dense breast tissue--it is a risk factor for breast cancer development, and it hides breast abnormalities, including cancer, making it the enemy of early detection.

Currently, no method of measuring breast density has been accepted as the gold standard. Over time, researchers will develop such a gold standard and they we will be able to quantify that density. And finally, research studies will nail down the exact relationship between breast density and a woman’s risk of developing breast cancer in the future.

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