Dense breasts have less fatty tissue and more non-fatty tissue compared to breasts that aren't dense.
One way to measure breast density is the thickness of tissue on a mammogram. The BI-RADS (Breast Imaging Reporting and Database System), which reports the findings of mammograms, also includes information on breast density. BI-RADS classifies breasts as one of four groups:
- mostly fatty
- scattered areas of density
- consistently dense
- extremely dense
Still, no one method of measuring breast density has been agreed upon by doctors. Breast density is not based on how your breasts feel during your self-exam or your doctor's physical exam. Dense breasts have more gland tissue that makes and drains milk and supportive tissue (also called stroma) that surrounds the gland. Breast density can be inherited, so if your mother has dense breasts, it's likely you will, too.
Research has shown that dense breasts:
- can be twice as likely to develop cancer as nondense breasts
- can make it harder for mammograms to detect breast cancer; breast cancers (which look white like breast gland tissue) are easier to see on a mammogram when they're surrounded by fatty tissue (which looks dark).
About 43% of women ages 40 to 74 years old in the United States are classified as having dense breasts.
As of September 2015, 24 states have passed legislation requiring that women be notified of their breast density with mammography results.
A study has found that breast density classifications can be unreliable -- as many as 22% of women had their breast density change from dense to not dense from mammogram to mammogram.
The study was published online on Jan. 12, 2016 by the Annals of Internal Medicine. Read the abstract of "Supplemental Screening for Breast Cancer in Women With Dense Breasts: A Systematic Review for the U.S. Preventive Services Task Force."
To do the study, the researchers systematically reviewed 24 studies from seven countries on breast density. Unfortunately, only six of the studies were considered to be good quality. Many of the studies were small and the results were not definitive.
Focusing on the good quality studies, the researchers found that even these studies showed that between 13% and 22% of women were reclassified as having dense or not dense breasts from mammogram to mammogram. One study found that different radiologists assigned the same BI-RADS breast density category to a particular woman only 82% of the time. This means that 18% of the time her breasts had a different density classification.
"The number of states mandating that breast density information be reported to patients is increasing," said Joy Melnikow, M.D., M.P.H., director of the Center for Healthcare Policy and Research at UC-Davis and first author of the paper. "But the assignment of breast density is not terribly consistent. The policy may be out in front of the science.
"It is important to be clear who actually has dense breasts," Dr. Melnikow continued. "Also, when patients are told their breasts are either dense or not dense, they need to have confidence in that assessment."
She believes the best solution for these issues is to conduct long-term, rigorous research to better standardize breast density classification.
If your mammogram reports consistently say that you don’t have dense breasts or that you do have dense breasts, then that is likely to be accurate. Still, if your breast density varies from mammogram to mammogram, you might want to talk to your doctor or the radiologist who read the mammogram to get a better understanding of your breast density.
"Evaluation of breast density is subjective at this point in time, and some patients, particularly women with breasts in the middle of the density spectrum, may find that their assessment varies from year to year," said Jennifer Tobey, M.D., clinical assistant professor of radiology at the University of Pennsylvania. "Changes in density assessment may also occur due to other factors such as weight loss or gain and as a result of aging. For a patient with a change in her density assessment from one year to the next, a conversation with her doctor may offer an explanation and clear up confusion. If there are remaining questions, and if the patient would like to have additional screening if her breasts are dense vs non-dense, the interpreting radiologist can be asked to review the images and discuss the choice of density category.
"Professionals involved in breast health understand how important it is to make breast cancer screening as approachable as possible for patients," she added. "While the breast density debate is currently a source of some confusion, hopefully in the near future density measurement will be more of an exact science and management options will be more straightforward."
If you know that you have dense breasts, there are lifestyle choices you can make to keep your risk of breast cancer as low as it can be, including:
- maintaining a healthy weight
- exercising every day
- limiting or avoiding alcohol
- eating a healthy diet full of fresh, whole foods and avoiding processed foods with a lot of added sugar and salt
- never smoking (or quitting if you do smoke)
- breastfeeding, if you have the option to do so
Along with lifestyle options, many doctors recommend additional screening for women with dense breasts. This might include MRI and ultrasound.
For more information on steps you can take to keep your breast cancer risk as low as it can be if you have dense breasts, visit the Having Dense Breasts page in the Breastcancer.org Lower Your Risk section.