The breast contains ducts, glands, fibrous connective tissue, and fatty tissue. Breasts that are dense or very dense have a lot of fibrous, connective tissue and less fatty tissue. Breasts that are not dense contain almost all fatty tissue and very little fibrous tissue. Virtually all women start out with very dense breast tissue. Over time this changes, and, as a woman ages, her breasts typically become more fatty. However, there are some women whose breasts remain fibrous and dense, even after menopause.
A doctor cannot tell by looking at a woman’s breasts how dense they are. Breast density can only be determined by mammography, with the radiologist evaluating the density seen on the X-ray according to four categories determined by the BI-RADS system. These categories are:
- A: Entirely fatty
- B: Scattered areas of fibroglandular density
- C: Heterogeneously dense, which may obscure masses
- D: Extremely dense, which lowers mammography sensitivity
Studies suggest close to half of all women between age 40 and 74 have dense breasts tissue. About 40% of these women have heterogeneously dense breast tissue (category C) and about 10% have extremely dense breast tissue (category D).
For decades, doctors have known it is much more difficult to find a cancer in women who have dense breasts. Dense breast tissue and a tumor will both appear white on a mammogram, making efforts to find a tumor akin to looking for a polar bear in the snow.
Then, in 2007, researchers reported findings that suggested breast density not only made it harder to find cancers, but was also was as an independent predictor of breast cancer risk. It now appears that women with heterogeneously dense breast tissue (category C) have about a 1.2 times greater risk of developing breast cancer than women with average breast density. Women with extremely dense breast tissue (category D) have about twice the risk.
In response to these findings, breast cancer advocates’ efforts have led 34 states to pass laws requiring doctors to provide women with information about their breast density along with their mammography results. Some laws also require women to be told to have additional screening with MRI or ultrasound. However, there is currently no evidence that more or different types of screening for women with dense breast tissue improves survival.
This legislation has led many women to become increasingly concerned about their breast density. But what many women don’t know is that it’s quite normal for women in their forties to have dense breast tissue. It’s breast density after menopause that’s more of a concern. Many articles about breast density also give the impression only women with dense breast tissue have tumors that are not seen on a mammogram and diagnosed at a later stage. But that's not the case.
As more research is done on breast density, new findings are continually reshaping our understanding of the link between density and breast cancer. For example, a study published in 2012 in the Journal of the National Cancer Institute, looking at 9,232 women who had been diagnosed with breast cancer between 1996 and 2005, found no relationship between BI-RADS density and breast cancer death. This finding raised questions about whether dense tissue may represent a local microenvironment promoting cancer development, but not its spread, and whether dense breast tissue increases the risk of getting cancer, but not dying of it.
More recently, findings from a study published May 19, 2015, in the Annal of Internal Medicine suggested breast density in and of itself should not be a sign that additional screening is warranted. The study confirmed women with dense breast tissue had a higher risk of breast cancer. However, the magnitude of risk also depended on other known risk factors. Based on this study, women with extremely dense breast tissue who also have a high five-year risk (2.5% or greater) of being diagnosed with breast cancer can benefit from additional imaging with ultrasound or MRI. But for women who have dense breast tissue and a low five-year risk, additional screening not only doesn’t find more cancers but also results in unnecessary biopsies.
- BreastDensity.info - A website developed by the California Breast Density Information Group to help patients understand breast density legislation.
- DenseBreast-info.org - An educational website about breast density.
- AreYouDense.org - A nonprofit organization advocating for breast density legislation.
- Breast Density Notification Laws by State - An interactive map developed by DenseBreast-info.org
Frequently Asked Questions
What can I do reduce my breast density?
Breast density is a measure of the composition of the breast. Women with dense breasts have more fibrous tissue and less fat, while those with less dense breasts have less fibrous tissue and more fat.
Many people think breast density is inherited—which would mean there is little that you can do about it. Breast density usually decreases after menopause as your breasts, realizing that they don't have to make milk anymore, go into retirement (involution). However, in some women this involution does not occur or it occurs much more slowly. It is in these postmenopausal women with dense breast tissue that breast cancer risk is higher.
I have very dense breast tissue. Should I take tamoxifen?
Preliminary data suggest that postmenopausal women with dense breasts respond well to the chemoprevention drug tamoxifen. One study on breast cancer risk found that women whose breasts became less dense while taking tamoxifen had a decreased risk of developing breast cancer than those whose breasts did not change while they were on tamoxifen. This finding supports other studies which suggest hormones have something to do with density—but exactly how is still unknown.
Because you have very dense breast tissue, you may want to be seen at a breast cancer clinic for high-risk women. Breast specialists at these clinics can help you assess your risk and discuss with you whether you should consider some type of breast cancer risk reduction.
Should I have a mammogram every six months because I have dense breast tissue?
There is no evidence that more frequent screening (more than once a year) is beneficial solely because a woman has dense breast tissue. However, the type of mammography performed may make a difference. Studies have found that digital mammography appears to be slightly better than film mammography at detecting cancer in women with dense breast tissue. So, finding a provider who uses a digital machine should be a starting point. The next thing you should do is talk with your doctor about whether you are at higher than average risk for breast cancer for reasons other than your breast density. You can use this online risk assessment tool to help you assess your risk as well. With this information in hand, you can better determine how frequently you should have mammography screening and whether the mammography screening should be supplemented with other forms of screening.