Your breasts are made up of fibrous tissue (or connective tissue), glandular tissue (the type of tissue that produces milk), and fatty tissue. If you're told you have dense breasts, this means that you have more fibrous and glandular tissue and less fatty tissue than women who don't have dense breasts. That’s why dense breast tissue is sometimes also called “fibroglandular tissue.”
Having dense breasts is normal; it is not a medical condition itself, and it does not cause symptoms. You can’t tell whether or not you have dense tissue by feeling the breasts. If you have firmer breasts, for example, this doesn’t necessarily mean you have dense breasts.
Dense breast tissue can only be seen on a mammogram. While fatty tissue appears dark on a mammogram, dense tissue appears white. For about half of women, screening mammograms reveal they have breast tissue that is categorized as dense. 1
Knowing whether you have dense breasts is important because:
Dense breasts make it harder for doctors to see breast cancers on mammograms. This increases the risk that cancers will be missed.
Women with dense breasts have a higher risk of developing breast cancer compared to women who don’t have dense breasts. The greater the amount of dense tissue, the higher the risk. However, you don’t necessarily have a high risk of breast cancer *just* because you have dense breasts. Breast density has to be considered along with other risk factors, such as age, family history, and any history of breast biopsies showing atypical cells or other changes that increase cancer risk.
Breast density is one piece of the puzzle in thinking about your breast health and breast cancer screening plan.
A: A tumor (circled) in a fatty breast is easily seen.
B: A tumor (circled) in a dense breast is difficult to detect.
Image copyright: DenseBreast-info.org and Dr. Wendie Berg
How is breast density measured?
When you have a mammogram, a radiologist reads the results using the Breast Imaging Reporting and Data System, or BI-RADS, published by the American College of Radiology. This is a standard system for reporting what’s seen on the imaging.
BI-RADS uses an assessment scale from 1 through 6 to indicate whether there were no unusual findings or, if something was found, whether it was more likely benign (not cancer) or malignant (cancerous). The report will also give a recommendation for routine screening or indicate what follow-up tests may be needed. An assessment of “0” means that additional imaging is first needed in order to characterize a potential finding.
In the BI-RADS report, the radiologist also includes a score for breast density on a scale from A through D:
A) Mostly fatty: The breasts are made up of mostly fatty tissue and contain very little fibrous and glandular tissue. About 10% of women have fatty breasts.
B) Scattered fibroglandular densities: The breasts are mostly fatty tissue, but there are a few areas of fibrous and glandular tissue visible on the mammogram. About 40% of women have scattered density.
C) Heterogeneously dense: A mammogram shows many areas of fibrous and glandular tissue. About 40% of women get this result.
D) Extremely dense: The breasts have large amounts of fibrous and glandular tissue. About 10% of women fall into this category. 1
A: Mostly fatty
B: Scattered fibroglandular densities
C: Heterogeneously dense
D: Extremely dense
Image copyright: DenseBreast-info.org and Dr. Wendie Berg
Women with a score of C or D are classified as having dense breasts. The radiologist makes the classification, so it often requires a judgment call — and studies suggest that two radiologists may classify the same woman differently. 2For this reason, many breast imaging centers are now using automated software to assist with evaluating breast density.
In the past, women typically didn’t receive information about breast density as part of their mammogram results. This has been changing over the past decade or so.
Why does breast density matter?
Dense breasts make it harder for radiologists to detect breast cancers when they read a mammogram. Cancers typically show up as small white spots or masses on a mammogram. Dense breast tissue also appears white on a mammogram. Small areas of cancer can hide behind the dense tissue, and it’s challenging to tell the difference between normal, healthy tissue and abnormal growths. The organization DenseBreast-info.org compares it to “trying to see a snowman in a blizzard.” Fatty breast tissue appears dark on a mammogram, so areas of concern that show up white are much easier to see.
Mammograms can miss about half of cancers in women with dense breasts. 3 4In addition, women with dense breasts are more likely to be diagnosed with breast cancer within the year after receiving a normal mammogram result, usually based on symptoms such as a lump or other breast changes. (This is called an “interval cancer.”)
Apart from hiding cancers on mammograms, dense breast tissue itself is associated with a higher risk of breast cancer. Doctors aren’t sure exactly why. Cancers develop in glandular tissue: the more glandular tissue there is, the greater the risk. Fibrous tissue may also produce growth factors that cause glandular tissue cells to divide and reproduce more than cells in fatty tissue do. Every time a cell divides, there is an opportunity for a “mistake” in the DNA to occur in the new cells — and multiple mistakes can eventually result in cancer.
Generally, the greater the amount of dense breast tissue you have, the higher your risk of breast cancer. It’s important to remember that most women are in the middle two categories of breast density (category B or C). For the 10% of women with extremely dense breast tissue (category D), breast cancer risk is about 2 times greater than for women who have scattered fibroglandular density (category B). For women who have heterogeneously dense breasts (category C), the risk of cancer is about 1.5 times that of a woman with scattered fibroglandular density (category B). These estimates are for women in general, though, and they don’t take other personal risk factors into account. Breast density alone isn’t enough to put you into a high-risk category for breast cancer.
“Risk in and of itself is multifactorial: it depends on other risk factors you might have in addition to dense breast tissue,” says Maxine Jochelson, MD, director of radiology at the Breast and Imaging Center at Memorial Sloan Kettering Cancer Center in New York City. “If breast density is your only risk factor, that’s one thing, but if there are additional risk factors, that’s different.”
If you have dense breasts, you and your doctor can discuss your individual risk level based on the amount of dense tissue you have. But you also should consider other breast cancer risk factors such as your personal health history — including radiation exposure to the chest, previous biopsies showing high-risk lesions, and/or whether or not you have a genetic mutation that increases cancer risk. You also should review any family history of breast cancer and other cancers.
How to find out if you have dense breasts
In the United States, 37 states and Washington, D.C., require breast imaging centers to give women some level of information about breast density after their mammogram. In 2019, the U.S. Food and Drug Administration (FDA) began to develop a single national reporting standard imaging centers can use to notify both the woman and her doctor about her breast density. However, this hasn’t yet been implemented. 4
Federal law requires that all women receive a letter notifying them about the results of their mammogram. In most states, this form letter now also tells you if you have dense breasts, and, in some states, your specific density category. However, the content of the letter varies widely by state and may not include specific details about your situation. Most of the letters will advise you to discuss your results with your doctor. Your breast density is also included in the final mammography report to your healthcare provider, prepared by the radiologist who read the images. If you live outside the United States, you may or may not be notified about your breast density, depending on the regulations in your country. DenseBreast-info.org is a good resource for finding out what the laws are where you live.
Whether or not you are notified about breast density after your mammogram, you can ask the doctor who ordered your imaging for a copy of your full report. If you’re using an electronic medical record, check there first. You’ll want to see the report written by the radiologist who read your mammogram, not just a “form letter” overview of your results. Read it over and look for information about breast density. Look for the terms “mostly fatty” or “scattered fibroglandular density” (meaning your breasts are not dense) vs. “heterogeneously dense” or “extremely dense” (which are dense breasts). The report also may include the letter rating from A through D, with a result of C or D meaning you have dense breasts. If you can’t get the report or you have trouble reading it, ask your doctor for the words used to describe your breast density.
Having dense breasts isn’t your fault, and it isn’t something you can control. Breast density is thought to be inherited in part, although the amount of dense breast tissue you have can change over time. Breast density can decrease as you go through menopause.
Some women are more likely to have dense breasts, such as those who:
use postmenopausal hormone replacement therapy (HRT)
have a lower body mass index (BMI)
And some women are less likely to have dense breasts, such as those who:
have gone through menopause
have had children
use the hormonal therapy tamoxifen, either to lower breast cancer risk or to treat breast cancer; aromatase inhibitors, which are another type of hormonal therapy, may also decrease breast density, but the impact doesn’t appear to be as substantial
But only your mammogram can show whether or not you have dense breasts.
Cancer screening for women with dense breasts
Mammograms don't always reveal cancers that may be present in dense breasts. Small cancers can be hidden by dense tissue, since both appear white on a mammogram. The more density there is, the harder it is for a radiologist to see an abnormality. Know your breast density and make it part of a conversation with your doctor about your annual screening plan.
Newer technology known as digital breast tomosynthesis (DBT), sometimes called three-dimensional (3D) mammography, appears to be more accurate than traditional 2D mammography for women with heterogeneously dense breasts. Like traditional mammography, 3D mammography takes two views of each breast, but then creates images of thin slices of the breast tissue for the radiologist to read. These slices are somewhat like a CT scan result, showing slivers of the breast with the overlying tissue peeled away. This can allow the radiologist to see lesions hidden within the tissue and better characterize their appearance. Check to see if your imaging center offers 3D mammography, and if not, ask your doctor if you can switch to one that does. (If not, be sure to keep up to date with your traditional 2D mammograms.)
If you’re among the 10% of women with extremely dense breast tissue, talk to your doctor about having a second imaging test, such as ultrasound or breast MRI, in addition to mammography as part of routine screening. Some doctors also may recommend supplemental screening if you have heterogeneously dense tissue, especially if you have other breast cancer risk factors. Added screening for dense breasts isn’t yet considered the standard of care and isn’t done routinely, so you may need to bring this up with your doctor.
In the United States, not all insurance companies cover supplemental screening; it depends on the laws in your state. For example, here in Pennsylvania where Breastcancer.org is located, a new law signed by the governor in June 2020 will require insurance coverage (subject to copay/deductible) for supplemental screening in women who have extremely dense breast tissue. For women with heterogeneously dense breast tissue, supplemental screening will be covered if they have one other high-risk factor for developing breast cancer. The law will not go into effect until sometime in 2021. Other states require insurance coverage for supplemental screening for all women with dense breasts while others limit it to certain groups and certain types of testing.
Most states still do not require insurance coverage for supplemental screening; however, these tests will likely be covered (with copay/deductible) if ordered by a health provider. DenseBreast-info.org maintains a map of legislation by U.S. state and also has information for people in Europe. Talk to your doctor to see what he or she recommends if you’re concerned about breast density.
“I think if a woman is in the D category — with extremely dense breast tissue — she should know that the mammography alone is very limited in her situation as far as early breast cancer detection,” says Jennifer Harvey, MD, chair of University of Rochester Medical Center Imaging Sciences. “I feel they really should get supplemental screening, as they can benefit from it.”
You and your doctor should discuss your individual situation, including the amount of dense breast tissue you have and whether you have other breast cancer risk factors. You also should discuss the cost of supplemental screening, what options are available in your area, and whether those tests would be covered by your insurance.
Supplemental screening for dense breasts has pros and cons. The main benefit is that adding a second imaging test to your mammogram makes it more likely that your doctor will be able to detect an early breast cancer. It also may give you some peace of mind that you are doing more to find breast cancer early.
The main risk of supplemental screening is the need to have additional testing and biopsies that may turn out to be unnecessary. Imaging studies often identify areas of concern that turn out not to be cancer (known as false positives), leading many women to experience more callbacks, more imaging, and biopsies. The biopsy itself can cause some level of anxiety, as can waiting to see what the results reveal. Researchers are still trying to figure out whether the benefits of supplemental screening for all women with dense breasts would outweigh the risks of doing unnecessary biopsies.
“Right now, a lot of it comes down to personal preference,” says Dr. Harvey. “Do you want more testing and possibly undergo a biopsy that wasn’t really necessary, or do you want to risk not finding an early cancer? A lot of it comes down to where an individual’s risk aversion lies.”
Keep in mind that the risk of a “false-positive” callback is the highest the first time you have a specific imaging test. Over time, radiologists will be able to compare your images to see if there are any concerning changes. If you ever switch facilities, it’s important to bring any previous images with you.
If you decide to have supplemental screening, your options may depend on what your insurance covers and what’s available in your area. Unlike mammography, which is fully covered by insurance, supplemental imaging often counts toward your deductible and is usually subject to a copay, so you may face some out-of-pocket costs. Talk to your doctor about your level of risk and which test(s) make the most sense for you.
The most common options are:
Ultrasound: Ultrasound is the most widely available supplemental screening option for women with dense breasts. It uses high-frequency sound waves to create images of breast tissue. The images are created as the waves pass through and are reflected by the tissue.
The traditional approach is hand-held ultrasound, in which a technologist or breast imager runs the transducer (the device that produces the soundwaves) over the breasts. A newer technology is automated breast ultrasound (AUS), which uses an automated scanner to produce a 3D image of the whole breast. Some imaging centers now offer AUS for women with dense breasts.
“The benefit of automated breast ultrasound is that the 3D images can be stored and radiologists can look at changes in tissue over time,” says Dr. Harvey — unlike handheld ultrasound images, which give information in real time but where there may or may not be saved images of a specific area for comparison. Whatever imaging center you’re using, she adds, make sure the technologists have substantial experience with breast ultrasound.
Breast MRI: MRI, or magnetic resonance imaging, is a technology that uses magnets and radio waves to produce detailed 3D images of breast tissue. Before the test, you need to have a contrast solution (dye) injected into your arm through an intravenous line. Cancers will take in more of the contrast solution than surrounding normal tissue.
A large study of 40,000 women with extremely dense breasts in the Netherlands, called DENSE (Dense Tissue and Early Breast Neoplasm Screening), found that breast MRI was able to pick up some early breast cancers that had been missed after a mammogram returned normal results. However, breast MRI also had an 8% false-positive rate — meaning that it indicated a suspicious area that later turned out not to be cancer — and led to many biopsies that weren’t necessary.
Research is ongoing to determine the value of breast MRI for women with extremely dense breasts. Although breast MRI picks up more breast cancers than ultrasound, it is a much more expensive and involved test, and it may not be available in all areas.
A new version of breast MRI called abbreviated breast MRI (or fast breast MRI) is showing promise as a quicker, less expensive alternative. It takes only about 10 minutes vs. 45 minutes for traditional MRI. A study of about 1,500 women published in the Journal of the American Medical Association in 2020 found that abbreviated breast MRI was better at finding invasive cancers in dense breasts than 3D mammography (digital breast tomosynthesis). It is not yet widely available and is not covered by insurance. However, Dr. Harvey notes that some imaging centers are offering this fast breast MRI as a self-pay service. The cost typically ranges from $300 to $500.
Contrast-enhanced digital mammography: Contrast-enhanced mammography (CEM) is just like having a regular mammogram, except for being injected with contrast solution first (the same type used for CT scans). The contrast helps to highlight any abnormal cancer cells that may be present. Although not yet widely available, studies suggest that CEM may be better than mammography combined with ultrasound at finding cancers in dense breasts. Small studies also suggest that CEM may be just as good as MRI.
Dr. Jochelson notes that MRI and contrast-enhanced mammography are more accurate because they are “physiologic” or “vascular” tests. When cancers form, they develop new blood vessels to feed the tumor. These vessels are leaky, and when women receive the contrast solution, it can help reveal the area of cancer — sometimes before the actual breast tumor can be seen. Mammography and ultrasound are different because they are anatomic tests, meaning that they image the structures of the breast. Although ultrasound is a good supplemental imaging tool, she notes, it can still miss many early breast cancers in dense tissue. She and her team at Memorial Sloan Kettering have led studies on contrast-enhanced digital mammography and are excited about its potential as a lower-cost alternative to MRI.
Although traditional ultrasound tends to be widely available, the other tests listed might only be available at larger health systems and academic medical centers.
Research continues to evaluate the best screening options for women with dense breasts. In the meantime, you can work with your doctor to figure out the right course of action for you — weighing your overall risk level, personal preferences, and the availability and cost of additional imaging. For women considered to be at high risk, usually due to a strong family history and/or specific genetic mutations, combined screenings with breast MRI and mammography are recommended starting at age 30.
Other steps you can take
If you have dense breasts, there are other steps you can take to care for your breast health. In addition to working with your doctor to consider supplemental screening, you can:
Make sure you have a mammogram every 12 months without delay, and try to go to a center with 3D mammography (digital breast tomosynthesis) if possible.
Perform breast self-exams so that you’re aware of any changes in your breasts, which should be reported to your doctor.
Follow all of the lifestyle recommendations for reducing breast cancer risk, such as:
Visit Risk and Risk Factors for more information.
National Cancer Institute. Dense Breasts: Answers to Commonly Asked Questions. Available at: https://www.cancer.gov/types/breast/breast-changes/dense-breasts
Melnikow J, Fenton JJ, Whitlock EP, et al. Supplemental Screening for Breast Cancer in Women With Dense Breasts: A Systematic Review for the U.S. Preventive Service Task Force [Internet]. Rockville (MD): Agency for Healthcare Research and Quality (US); 2016 Jan. (Evidence Syntheses, No. 126.) Available at: https://www.ncbi.nlm.nih.gov/books/NBK343793/
Kolb TM, Lichy J, Newhouse JH. Comparison of the performance of screening mammography, physical examination, and breast US and evaluation of factors that influence them: An analysis of 27,825 patient evaluations. Radiology 2002; 225:165-175. Available at: https://doi.org/10.1148/radiol.2251011667
Berg WA, Zhang Z, Lehrer D, et al. Detection of breast cancer with addition of annual screening ultrasound or a single screening MRI to mammography in women with elevated breast cancer risk. JAMA 2012; 307:1394-1404. Available at: https://dx.doi.org/10.1001%2Fjama.2012.388
O’Connor M. Health Imaging. New legislation mandates nationwide breast density notification. Available at: https://www.fda.gov/news-events/press-announcements/fda-advances-landmark-policy-changes-modernize-mammography-services-and-improve-their-quality
— Last updated on February 3, 2022, 8:18 PM