Dense breasts have less fatty tissue and more non-fatty tissue compared to breasts that aren’t dense. Breasts that are dense have more gland tissue that makes and drains milk and supportive tissue (also called stroma) that surrounds the gland.
It’s estimated that about 45% of all women have dense breasts.
Having dense breasts has been linked to a much higher risk of breast cancer. At the same time, having dense breasts also can make it harder for mammograms to detect breast cancer. Breast cancers, which look white on a mammogram, like breast gland tissue, are easier to see on a mammogram when they’re surrounded by fatty tissue, which looks dark on a mammogram.
It’s recommended that women with dense breasts have more frequent screening, including screening with MRI and/or ultrasound in addition to mammograms. Twenty-one states have laws requiring women be notified if they have dense breasts and that the women be advised to talk to their doctor about additional screening tests.
A study suggests that only about half of women with dense breasts have a higher-than-average risk of breast cancer. The researchers said that women and their doctors should consider other risk factors in addition to breast density when making decisions about screening plans.
The study was published online on May 18, 2015 by the Annals of Internal Medicine. Read the abstract of “Identifying Women With Dense Breasts at High Risk for Interval Cancer: A Cohort Study.”
In the study, the researchers looked at 9 years’ worth of records for 365,426 women ages 40 to 74 who had 831,455 screening digital mammograms from 2002 to 2011. The information came from the Breast Cancer Surveillance Consortium mammography registries.
The researchers wanted to know if there were any groups of women with dense breasts who had a higher risk than other women with dense breasts of developing an interval breast cancer.
An interval breast cancer is a cancer that is found less than a year after a normal mammogram. Interval cancers are usually found when a woman or her doctor feels a lump. Interval cancers may have been present during the previous mammogram, but not detected. Interval breast cancers are considered more aggressive than cancers that are found by a routine yearly or every-other-year mammogram.
The researchers said that women at high risk for interval cancers are likely to benefit the most from additional screening tests such as MRI or ultrasound.
The researchers looked at three risk factor measurements:
- the women’s breast density
- the Breast Cancer Surveillance Consortium (BCSC) 5-year risk model score for each woman
- this model considers a woman’s age, how many first-degree relatives have been diagnosed with breast cancer, biopsy history, race/ethnicity, and breast density
- a 5-year risk score of 1.66% or lower is considered low to average, a score above 2.50% is considered high, and a score above 4% is considered very high
- the number of women who developed interval cancers during the study
The results showed that two groups of women had the highest risk of interval cancers:
- women with a BCSC 5-year risk score of 1.67% or higher and extremely dense breasts; this group included 47.5% of women with extremely dense breasts
- women with a BCSC 5-year risk score of 2.50% or higher and heterogeneously dense breasts (heterogeneously dense means more than half of the tissue in the breast is dense); this group included 19.5% of women with heterogeneously dense breasts
Both of these groups had an interval cancer rate of more than 1 case per 1,000 mammograms, which is considered high.
Together, these two groups of women made up 24% of women with dense breasts and 12% of the women in the study. According to the researchers, this means that about a quarter of the women with dense breasts would benefit from additional screening with MRI or ultrasound.
More than half of the women with extremely dense or heterogeneously dense breasts had low to average BCSC risk scores:
- 52.5% of women with extremely dense breasts had a BCSC score between 0% and 1.66%; the interval cancer rate for this group was 0.72 to 0.89 case per 1,000 mammograms
- 51.0% of women with heterogeneously dense breasts had a BCSC score between 0% and 1.66%; the interval cancer rate for this group was 0.58 to 0.63 case per 1,000 mammograms
The researchers suggested that women with dense breasts and their doctors use the BCSC 5-year risk calculator to determine a risk score and then talk about whether more screening tests make sense for that woman’s individual situation.
The researchers also gave examples of how the information from the study might be used. If 100,000 women with dense breasts all had additional screening tests with MRI or ultrasound, about 89 interval cancers would be found. This means it would take 1,124 tests to find one cancer.
If the additional screening were given only to the two groups of women with the highest risk of interval cancer as found in the study, 24,294 of the 100,000 women would have the additional screening tests and about 35 interval cancers would be found. This means that it would take 694 tests to find one cancer.
Still, this means that about 54 cancers wouldn’t be found if the entire group of 100,000 women didn’t have additional screening tests.
“You can imagine, someone may not like that strategy,” said Dr. Karla Kerlikowske in an interview in the New York Times. Dr. Kerlikowske is the first author of the study and a professor of medicine and epidemiology and biostatistics at the University of California, San Francisco. “You’re only targeting higher-risk women. My argument to that is the people in the high-risk, high-density group are at high risk of advanced cancer, so you’re more likely to benefit those women.”
Other experts said that while the study offers interesting information, it doesn’t really offer a definitive answer about whether all or just some women with dense breasts should have additional screening tests.
If you know you have dense breasts, it makes sense to talk to your doctor about this study. It also makes sense to sit down with your doctor and use the BCSC calculator to determine your 5-year risk score.
If you have a higher-than-average risk of breast cancer because you have dense breasts and/or because of another risk factor, 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:
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 mammograms, ultrasound, and other tests used to detect breast cancer, visit the Breastcancer.org Screening and Testing pages.
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