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 6 times more likely to develop cancer
- 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.
Because mammograms don’t always find cancers in dense breasts, researchers have been studying the effectiveness other screening methods for dense breasts.
A study has found that adding 3-D mammography (also called digital tomosynthesis) or breast ultrasound to regular screening mammograms can detect more cancers in dense breasts. Ultrasound was slightly better at detecting cancers in dense breasts than 3-D mammography and both screening methods had similar false-positive rates.
The study was published online on March 9, 2016 by the Journal of Clinical Oncology. Read the abstract of “Adjunct Screening With Tomosynthesis or Ultrasound in Women With Mammography-Negative Dense Breasts: Interim Report of a Prospective Comparative Trial.”
The study also was presented on March 9, 2016 at the 2016 European Breast Cancer Conference. Read the abstract of “Interim results of the Adjunct Screening with Tomosynthesis or Ultrasound in Mammography-negative Dense Breasts (ASTOUND) trial.”
A false-positive is when a screening test finds an area that looks like cancer but turns out to be normal. Besides worrying about being diagnosed with breast cancer, a false positive means more tests and follow-up visits, which can be stressful and scary. Still, other research has found that while false positive results do cause anxiety and stress, the anxiety is short-term and doesn’t affect a woman’s overall health and well-being.
The ASTOUND study was started in December 2012 and included 3,231 women with breasts classified by BI-RADS as either consistently dense or extremely dense. The women were 44 to 78 years old, and screening mammograms had found no cancer in their breasts.
After their last regular screening mammogram, the women also had a 3-D mammogram and breast ultrasound.
The 3-D mammograms and ultrasounds were read by different radiologists who knew that the standard screening mammogram had found no cancer, but didn’t know the result of the other additional screening test.
The additional screening tests found 24 breast cancers:
- 12 were detected by both 3-D mammogram and ultrasound
- one was detected only by 3-D mammogram
- 11 were detected only by ultrasound
"These results mean that tomosynthesis detected an additional four breast cancers per 1,000 women screened and ultrasound detected an additional seven breast cancers per 1,000," said Dr. Nehmat Houssami, professor of public health at the University of Sydney, who presented the study at the European Breast Cancer Conference.
There were 107 false-positive results:
- 53 for 3-D mammograms
- 65 for ultrasound
This difference wasn’t statistically significant, which means that it was likely due to chance and not because of the difference in screening methods.
In an editorial in the Journal of Clinical Oncology that ran with the paper, Wendie Berg, M.D., of the Magee Women’s Hospital at the University of Pittsburgh Medical Center, wrote that a high false-positive rate is why many doctors have been reluctant to use ultrasound for breast cancer screening. The ASTOUND study may start to change that.
“Importantly, in preliminary results from the ASTOUND trial, false-positive recalls (2.0%) and biopsies (0.7%) were acceptably low,” Dr. Berg wrote.
"In this study, we are comparing two additional tests to see if they can do better than standard mammograms in finding cancer in women with dense breasts; we have found that ultrasound does better than tomosynthesis, but ultrasound is a separate test, it is time-consuming and, in less experienced hands, it can lead to a lot of false alarms,” Dr. Houssami said. “However, tomosynthesis, which is a form of refined mammography, can be carried out as part of the standard 2-D mammogram screen, or even instead of it. Given that tomosynthesis detected more than 50% of the additional breast cancers in these women, the implications are that it has the potential to be the primary mammography screening method without the need for an extra screening procedure.
“…Our study does not provide all the answers on this issue but provides the first critical piece of information on how these two tests compare,” Dr. Houssami added. “If a woman is concerned that her breasts are very dense on the mammogram (or has been told her breasts are very dense and would like more testing), I can use the data from ASTOUND to discuss with her the option of having the ultrasound or the tomosynthesis screen; I would discuss with her the pros and cons of adding another test to improve sensitivity for detecting cancer, but would also point out this could have additional harms such as more false alarms."
If you have dense breasts, you and your doctor will develop a screening plan tailored to your unique situation. General recommended screening guidelines include:
- a monthly breast self-exam
- a yearly breast exam by your doctor
- a digital mammogram every year starting at age 40
Digital mammography is better than film mammography in women with dense breasts, regardless of age.
Your personal screening plan also may include:
- 3-D mammogram
Talk to your doctor about developing a specialized program for early detection that meets your individual needs and gives you peace of mind.
To get the best information from your breast imaging studies, it's important to:
- compare this year's study to prior years' studies
- correlate the information from the imaging studies (what you feel in the breast compared to the results of your mammogram, MRI, and ultrasound)
Usually test reports will say if the most recent imaging test is different from other tests or earlier results from the same test.
To keep track of your test results, you may want to get a copy of each imaging report and put them in a binder.
For more information on 3-D mammograms, ultrasound, and other tests used to detect breast cancer, visit the Breastcancer.org Screening and Testing pages.