You may have heard health reports about the importance of breast density on a mammogram (also called mammographic density), which has emerged as a strong risk factor for breast cancer in women. But what, exactly, is breast density? What is its role in breast cancer? How do you know if you have dense breasts? And, if you do have dense breasts, is there anything you can do to lower your breast cancer risk?
A woman’s breasts are made up mostly of fat and breast tissue. Breast tissue is the network of lobules (sacs that produce milk) and ducts (canals that carry milk from the lobules to the nipple openings during breastfeeding). Connective tissue helps hold everything in place.
What is breast density?
Breast density is a way to describe the composition of a woman’s breasts. This measure compares the area of breast and connective tissue seen on a mammogram to the area of fat. Breast and connective tissue are denser than fat and this difference shows up on a mammogram.
- High breast density means there is a greater amount of breast and connective tissue compared to fat.
- Low breast density means there is a greater amount of fat compared to breast and connective tissue.
How is breast density measured?
Currently, there are several ways to measure breast density. All of these measures rely on a physician’s visual assessment of the mammogram. Thus, these assessments are somewhat subjective, and one physician’s estimate of breast density may be different from another’s. When the mammogram shows a breast is very dense or very fatty, agreement is high. However, when the mammogram shows something in between, agreement tends to be lower.1 This is an issue with all current measures of breast density.
The most common method is the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS®) for breast density. This system, however,is not routinely reported or used by health care providers to assess breast cancer risk. The best way to measure breast density remains an active area of research.
How do I know if I have dense breasts?
After looking at your mammogram, the radiologist (the physician who interprets your mammogram) may record breast density using BI-RADS® or a similar measure. Using this measure or by looking at your mammogram itself, your provider may conclude that you have dense breasts. If your mammogram report does not include information about the radiologist’s assessment of your breast density, you may want to ask your provider for this information.
Breast density and breast cancer risk
High breast density, as seen on a mammogram, is linked to an increased risk of breast cancer. Women with very dense breasts are four to five times more likely to develop breast cancer than women with low breast density.2-3 According to Ann Partridge, MD, MPH, Clinical Director of the Breast Oncology Center at Dana-Farber Cancer Institute and Associate Professor of Medicine at Harvard Medical School, “breast density is clearly a risk factor for breast cancer which we are now figuring out how to use to help women better understand their individual risk and hopefully will someday be a risk factor that we can modify to reduce risk.”
At this time, we do not understand why breast density is related to breast cancer. Researchers are looking into many possible mechanisms in the body that might explain this relationship.
Factors related to breast density
Many factors related to breast density are also related to breast cancer. Learning more about these factors may help explain how breast density increases breast cancer risk. It may also provide clues for ways to lower breast cancer risk in women with dense breasts.
Some of the factors shown to be related to breast density are discussed below. All of these factors and their role in breast density are still under study.
Having dense breasts appears to run in families and is likely related to some genetic factors. Specific genes that might be linked to breast density are under study.4-7
Early life exposures
Growth and development in early life may impact breast density.8 For example, higher birthweight appears to be related to higher breast density in adulthood.9 And, higher body weight during adolescence may be related to lower breast density.10
Pregnancy and childbearing
Breast density decreases somewhat with each pregnancy. So, the more children a woman has given birth to, the less dense her breasts tend to be.4,11-12
Similarly, the more children a woman has given birth to, the lower her risk of breast cancer.13
Age and menopause
During menopause, hormone changes in the body cause the breast tissue to become less dense. So, in general, younger, premenopausal women have denser breasts than older, postmenopausal women. However, younger women who have gone through early menopause due to oophorectomy (removal of the ovaries) or total hysterectomy (removal of the uterus and the ovaries) may have lower breast density.
This is confusing: breast density decreases with age and yet, breast cancer risk increases with age. It may be that breast density plays an early role in the development of breast cancer, but at this time, we simply don’t know. As we learn more, these relationships should become clearer.
Postmenopausal hormone use (estrogen plus progestin)
Women who use postmenopausal hormones (estrogen plus progestin) have higher breast density than women who do not use these hormones. Their breast density decreases once they stop using hormones.14-15
Postmenopausal hormone use also increases the risk of breast cancer.16-19 Recent findings suggest women who have high breast density may have an added risk of breast cancer (beyond the risk due to high breast density) if they use postmenopausal hormones.20
Breast density is related to body weight. Compared to women with higher breast density, women with lower breast density are more likely to have:10
- Higher body weight in adulthood
- Higher body weight during adolescence
- Increased weight gain since age 18
Body weight affects breast cancer risk differently before and after menopause. Before menopause, being overweight offers modest protection against breast cancer in women. After menopause, being obese or overweight increases the risk of breast cancer.21-23
As with age, these differences are confusing: breast density is lower among women with a higher body weight, yet, after menopause, higher body weight increases breast cancer risk. Although we do not yet understand the relationship between body weight, breast density and breast cancer risk, this topic is under active study.
Screening for women with dense breasts
Mammography (digital mammography)
On a mammogram, fat in the breast looks dark and the denser breast and connective tissues look light gray or white. Because cancer can also appear white on a mammogram, it is harder to interpret mammograms in women with dense breasts.
Standard mammography produces an X-ray image on film, while digital mammography allows the image to be viewed on a computer screen. On the computer, certain sections of the X-ray image can be magnified and easily examined more closely, and the contrast of the image can be adjusted. This makes digital mammography better at finding tumors in women with dense breasts than standard film mammography.24
Magnetic resonance imaging (MRI), in combination with mammography, is under study as a breast cancer screening tool for women with dense breasts.25-26 Currently, the American Cancer Society (ACS) feels there is not enough evidence to recommend for or against MRI plus mammography screening for women with dense breasts. Rather, ACS suggests women with dense breasts talk to their health care providers about whether they should consider adding MRI to their annual mammography screening.25
Ultrasound, in combination with mammography, is also under study as a breast cancer screening tool for women with dense breasts.26
I have dense breasts, now what?
At this time, there are no specific recommendations on lowering breast cancer risk for women with dense breasts. Although women with dense breasts appear to be at higher risk of breast cancer, it is not clear that lowering breast density will necessarily decrease risk of breast cancer. For example, getting older and gaining weight after menopause are both related to a decrease in breast density, but are also related to an increase in cancer risk.
However, all women can take steps to lower their breast cancer risk (learn more about making healthy lifestyle choices).
There are no special breast cancer screening tests recommended for women with dense breasts. However, if you have dense breasts, talk to your health care provider about which breast cancer screening tests are right for you.