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. 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.”
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.
“…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.”
Bottom line: For women with dense breasts, the addition of 3D mammography or bilateral whole breast ultrasound improves the early detection of breast cancer.
About the author: Raja P. Reddy, MD is a board certified diagnostic radiologist specializing in breast imaging. He is also a contributing editor for Women’s Imaging Specialists, a leading provider of outpatient women’s imaging services in the greater Atlanta, GA area.