Dense Breasts

What does it mean to have “Dense Breasts”

Breast density is defined by the appearance of breast tissue on mammography, not how breasts feel on exam or how big they are. Dense breasts have more glandular elements and connective tissue, while less-dense breasts have more fatty tissue. On a mammogram, fatty tissue appears black, while “fibroglandular” or dense breast tissue appears white. Breast density is determined by several variables including patient age, genetics, menopausal status, weight, parity, and environmental factors. In general, younger women have more dense breast tissue than older post-menopausal women. Since over 50% of breast density is determined by genetics, age and even weight may not predict how dense a woman’s breasts are. Having dense breasts is common, with about half of all women being described as having “dense” breasts.

Classification of Breast Density

There are a number of methods to score breast density, however the most frequently used is the “BIRADS” breast density score. BIRADS stands for Breast Imaging-Reporting and Data System and is a method for standardizing reporting for mammography. The similar system is used to categorize breast density as it appears on mammography. The four categories, BIRADS 1-4, describe breast density as fatty, scattered fibroglandular, heterogeneously dense, and extremely dense, consecutively. Breasts that are described as BIRADS1 or fatty have less than 25% glandular elements and 75% of breast tissue appears black (for fat) on mammogram. Someone with scattered fibroglandular tissue has >25% and up to 50% dense breast elements, while women with heterogeneously dense breasts have >50% and up to 75% glandular elements. The last category, BIRADS 4, or extremely dense breasts, account for 10% of all women and includes women with >75% fibroglandular elements on mammogram. Women with BIRADS 3 and 4 breast density (heterogeneously dense and extremely dense) are considered to have “dense” breasts. This accounts for 50% of all women. Again, this scoring system is independent of a woman’s weight or breast size, so a very thin woman can have fatty breasts and an obese woman can have dense breasts.

Cancer Risk and Dense Breasts

There are two big issues with having dense breasts, even-though half of all women will be described as having dense breasts on mammogram. The first issue is that the more dense a breast is, the whiter it appears on mammogram. Unfortunately, cancer is also generally white on mammogram so in very dense breasts, cancer can be harder to detect. In fact, up to 30% of early breast cancers can be missed on a mammogram in women with extremely dense breasts. The second problem is that having dense breasts alone is an increased risk for developing breast cancer. Numerous studies have shown this, with risk ranging from 1.8 to 6-fold increased risk, on average about a four-fold increased risk has been most commonly reported. One of the most prominent studies on breast density risk was published in the New England Journal of Medicine in 2007 that showed that women with the highest density breasts, compared to those with fatty breasts, had a 3.5-fold increased risk of having a breast cancer detected on screening mammography and a 17.8-fold increased risk of developing an interval cancer—a cancer found before a woman’s next annual mammogram.

Screening Options

Since women with dense breasts on average have an increased risk of developing breast cancer and a harder time to have their cancer detected with traditional mammography alone, the most logical next question is what else is available for screening these “at risk” women. Fortunately there are some tools that can help. Screening breast ultrasound and automated whole breast ultrasound have been shown to aid in the detection of cancer in women with dense breasts. 3D-mammography, or tomosynthesis, has also been shown to be superior in detecting breast cancer in women with dense breasts as well as less-dense breasts. The most sensitive tool at detecting breast cancer remains MRI, however this is costly and requires contrast injection so should be reserved for those women at highest risk. Mammography should not be replaced with ultrasound or MRI since it can still be useful in detecting asymmetries and calcifications, however the additional modalities can be supplemental.

Breast Density Laws

Due to the accumulation of data showing the risk of cancer associated with extremely dense breasts, several states have created new legislation to mandate reporting of breast density to inform patients whether or not they have dense breasts. In California, this law, SB-1538, was enacted in 2013 and required that radiologists inform all women if they have dense breasts. Unfortunately, the California law does not give further guidelines on supplemental imaging nor does it require that insurance companies provide coverage for supplemental screening for women with dense breasts. What is recommended, however, is that these women discuss additional screening options with their physicians.

What is the right screening?

Since there is no “one size fits all” in determining best screening, it is important that women undergo a risk assessment to determine the best screening method for them. There are a number of programs and tools that can calculate a woman’s lifetime risk of developing breast cancer, the most common are the Gail model and the Tyrer-Cuzick model. These models compute risk based on a number of personal and family risk factors including age at menses, age at menopause, prior biopsies, and first degree relatives with breast cancer, to name a few.

Dr. Kapoor’s screening recommendations:

FattyScattered FibroglandularHeterogeneously DenseExtremely Dense
Average Risk*
15% or less
Intermediate Risk
High Risk >20%3D MMG and MRI3D MMG and MRI3D MMG and MRI3D MMG and MRI

*Lifetime risk based on Tyrer-Cuzick model

Can we decrease this density?

While approximately 50% of breast density is genetically pre-determined, there is possibly some room to manipulate and adjust our breast density. There are studies that suggest hormones can influence breast density and hormone replacement therapy has been shown to increase breast density. Conversely, however, studies looking at hormone blockade to decrease breast density are inconclusive. There is speculation that diet and alcohol can alter breast density, however there is also not sufficient evidence to support this either. This is certainly an area of active research, so stay tuned!

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