An Emerging Breast Cancer Risk Factor: Breast Density

Patient Expert

Do you have dense breasts? Only your radiologist knows for sure. And that could be an increasingly important piece of information, as evidence increases that breast density is one of the prime risk factors in breast cancer, particularly in pre-menopausal women.

What Is Breast Density, Anyway?

It's based on the amount of fat tissue in your breasts: the more fat tissue, the lower your breast density. Aside from cosmetic issues-so what? Well, breasts with more fat tissue are easier and more reliable candidates for mammography. A mammogram works by shooting x-rays through your breast, with the result recorded on film (or now, often, digitally). Breast tissue shows up as a light color; fat as dark. A tumor, which is the same density as breast tissue, will also show up light. Thus a light-colored tumor is much more easily spotted against the dark background provided by fat tissue, than against the similarly light background of breast tissue, where it can hide. Bottom line: The denser your breasts (the less fat they contain), the more easily they can disguise a tumor when you get your annual mammogram.

Research on Breast Density and Breast Cancer Incidence

Now, research is showing that not only are tumors harder to spot in women with dense breasts; these women also are at increased risk for breast cancer, no matter when it's detected. "After age, [breast density] is probably the most important factor [determining breast cancer risk]," said William E. Barlow, lead author of a study examining the relationship between breast density and breast cancer incidence, whose results were printed last fall in the Journal of the National Cancer Institute. "If we wanted to identify women who were really at high risk, for chemoprevention efforts or more intense screening surveillance, then any model that incorporates breast density is going to be better at picking out those women."

How Many Women Have Dense Breasts?

"Dense breasts" is a specific genetic condition that affects about one in six women. But it's a characteristic, as well as a genetic condition. There's a huge range in breast density among women, with some exhibiting nearly all fat tissue in their breasts, while others have nearly none. If you go to the far end of that scale, to women with extremely dense breasts, recent studies have shown that these women have about a four to six times greater risk of breast cancer as women whose breasts aren't dense. Scientists aren't yet sure why this is, beyond the fact that cancer occurs in breast tissue, not fat; so the higher your ratio of breast tissue, the more opportunity you have for cancer to develop.

How Dense Are YOUR Breasts?

Sounds like it would be a key piece of information for you to have, when you're assessing your risk factors for breast cancer. Alas, it's a determination that can only be made by your radiologist. As Barlow notes, "It's not something that a woman can judge for herself. There really isn't a feedback mechanism from the radiologist back to the woman to say what the breast density is." So, unless you make it your mission to contact your radiologist, ask the question, and get an answer, your breast density will remain a mystery. (And we know how challenging it can often be to pry specific personal information out of a complex health care system.)

As awareness of breast density as a predictor of breast cancer is raised, perhaps it'll become easier for a woman to find out her own personal breast density. Perhaps your benchmark mammogram at age 40 will include an assessment; and perhaps each mammogram after that will refer to your increasing or decreasing density. We can only hope.