BREAST DENSITY IS A MEASURE OF BREAST PROPORTIONS of fibrous and glandular tissue, compared with fat tissue. Today I want to explore breast density, breast cancer risk, and breast cancer detection. We begin with a brief quiz:
Which of the following statements is/are FACT?
- A. Breast cancer incidence is increased for those with a greater proportion of dense breast tissue on mammograms.
- B. Dense breast tissue can interfere with identifying cancer on mammograms.
- C. Both A and B.
You are correct if you guessed C (both A and B).
If a recent mammogram showed you have dense breast tissue, you might be concerned about what this means for your breast cancer risk. Today, we explore the implications of having dense breasts.
Breast density basics
Breast density measures the ratio of fibrous and glandular tissue (that is, fibroglandular tissue) in your breast, compared with fat tissue. Density has nothing to do with size or firmness.
Breasts are milk ducts, lobules, and fatty and fibrous connective tissue. The ducts are the tiny tubes carrying milk from the lobules to the nipple. Small glands known as lobules produce breast milk. The ducts and lobules together form glandular tissue.
On the other hand, fibrous tissue and fat give a breast its shape and size while holding other structures in place.
On a mammogram, non-dense breast tissue appears dark and transparent. The dense breast tissue appears solid white on a mammogram, making it challenging to see.
A radiologist analyzes your mammogram to determine the ratio of non-dense tissue to dense tissue and assigns a breast density level.
Breast density classification
Radiologists use the Breast Imaging Reporting and Data System (BI-RADS) to describe breast density. Density is often recorded in your mammogram report using letters:
- A: Almost entirely fatty. The breasts are almost entirely composed of fat. About one in 10 women has BI-RADS A breast tissue.
- B: Scattered areas of fibroglandular density. Some scattered areas of density exist, but most of the breast tissue is non-dense. About four in 10 women have this result.
- C: Heterogeneously dense. There are some areas of non-dense tissue, but most breast tissue is dense. About four in 10 women have this result.
- D: Extremely dense indicates that nearly all breast tissue is dense. Approximately one in 10 women has extremely dense breasts.
In general, we consider women with breasts that are heterogeneously or extremely dense to have dense breasts. About half of women have dense breasts.
Dense breast tissue may increase the risk of breast cancer
Mammograms identify breast density. Additional imaging tests are sometimes recommended for women with dense…
For an individual, breast density can change over time. Here are some factors that increase the chances of having dense breasts:
- Young age.
- Pregnancy or breastfeeding.
- Hormone replacement therapy (HRT).
- A lower body weight.
Breast density and cancer incidence
To determine the breast cancer risk associated with breast density among women aged 65 to 74 versus 75 or older, researchers prospectively examined data from the Breast Cancer Surveillance Consortium. They also wanted to see if body mass index (BMI) altered the association.
Here are the main outcomes of this study of over 220,000 mammograms:
The five-year cumulative invasive breast cancer incidence rose with increasing density.
For those aged 65 to 74, the breast cancer incidences by density class are as follows:
- Almost entirely fatty breasts — 11.3 per 1000 women
- Scattered fibroglandular densities — 17.2 per 1000 women
- Extremely or heterogeneously dense breasts — 23.7 per 1000 women
For those 75 or older, the results are as follows:
- Almost entirely fatty breasts — 13.5 per 1000 women
- Scattered fibroglandular densities — 18.4 per 1000 women
- Extremely or heterogeneously dense breasts — 22.5 per 1000 women
In summary, those with extreme or heterogeneous breast density have an increased breast cancer risk of breast cancer compared with scattered fibroglandular breast density.
Body mass index (BMI) did not modulate the associations between breast density and breast cancer risk.
Breast density and cancer detection
Breast density increases breast cancer risk. Moreover, dense breast tissue makes it more challenging to find cancer on a mammogram. Both dense breast tissue and cancer appear white on a mammogram. Nevertheless, mammograms can be effective and remain an important screening tool, irrespective of breast density.
Today, we do most imaging in the United States with digital mammograms — the rest images are saved as digital files (rather than film), allowing for more detailed analysis. Digital mammograms are more effective at finding cancer in dense breast tissue than traditional film mammograms.
Breast imaging — Are additional tests effective?
Tests added to mammograms can increase the chances of detecting cancer in dense breast tissue.
There’s some evidence that additional tests may make breast cancer more likely to be detected in dense breast tissue. But additional tests had added risks, and no additional test reduces breast cancer mortality.
Ask your doctor whether you need supplemental imaging based on your other breast cancer risk factors and personal preferences. The Mayo Clinic (USA) offers additional tests for breast cancer screening may include:
- 3-D mammogram (breast tomosynthesis). Tomosynthesis uses X-rays to collect numerous breast images from several angles. A computer combined the images to form a 3-D image of the breast. Many hospitals, including my own, use 3D-type mammograms exclusively.
- Breast MRI. MRI uses magnets (but no radiation) to make breast images. We recommend MRI for women with a very high risk of breast cancer, such as those with genetic mutations that increase cancer risk.
- Breast ultrasound. Ultrasound uses sound waves to analyze tissue. Radiologists typically use diagnostic ultrasound to investigate areas of concern discovered on a mammogram.
- Molecular breast imaging (MBI). MBI, also known as breast-specific gamma imaging, uses a special camera (gamma camera) that records the activity of a radioactive tracer. The radiology technologist injects a tracer into a vein in your arm. Normal and cancerous tissue react differently to the tracer, as seen in the gamma camera images. MBI is performed every other year in addition to an annual mammogram. Molecular breast imaging is not widely available.
Let’s look more at some additional tests:
Tomosynthesis (3D) mammograms detect about one additional cancer per 1000 women. The test lowers the chances of being called back for more testing of areas that aren’t cancer. As noted above, 3D mammograms can help evaluate dense breast tissue. 3D mammograms expose you to additional, very low levels of radiation.
Breast magnetic resonance imaging (MRI) can detect approximately 14 additional cancers per 1000 women. There is no radiation exposure, but MRI requires injection of contrast material. Another downside of MRI? It is more likely to find areas of concern that are not cancer but require additional imaging or a biopsy. In the USA, insurance may not cover the test unless you have a very high risk of breast cancer.
A new version of breast MRI is the abbreviated breast MRI (or fast breast MRI). This approach is promising as a more rapid, less expensive version of magnetic resonance imaging. A fast breast MRI takes about 10 minutes, compared with er minutes for a traditional MRI.
A study of approximately 1,500 women published in 2020 in the Journal of the American Medical Association reported that abbreviated breast MRI discovered more invasive cancers in dense breasts than in 3D mammography (digital breast tomosynthesis). Fast MRI is not yet widely available, and insurance does not cover it.
Breast ultrasound can detect an additional two to four cancers per 1000 women with no additional radiation exposure. Ultrasound can indicate areas of concern (that are not cancer), leading to unnecessary additional imaging or a biopsy. The ultrasound’s accuracy is highly dependent on the experience of the individual performing the test.
Breast density — My take
In summary, breast density is important for two main reasons:
- Women with dense breasts have an elevated breast cancer risk compared with those with less dense breast tissue.
- Dense breast tissue makes it more challenging for radiologists to see cancer on mammograms.
Please talk with your doctor about how often you should have breast cancer screening and whether you should consider any additional tests. Thank you for joining me today for this look at breast density, breast cancer risk, and screening.
Whether you have dense breasts, you can take some risk-reducing steps. Don’t forget to get any recommended mammograms without delay. If possible, try to go to a center with 3D mammograms (digital breast tomosynthesis). Know your breasts and report any changes to your health team.
Finally, don’t forget to follow all lifestyle recommendations for reducing breast cancer risk, such as maintaining a healthy weight, regular physical activity, limiting alcohol, eating nutritious food, never smoking (or quitting if you smoke), and getting adequate sleep.
Thank you for joining me in this look at breast density, cancer risk, and cancer detection.
The information I provided in this blog is for educational purposes only and does not substitute for professional medical advice. Please consult a medical professional or healthcare provider if you seek medical advice, diagnoses, or treatment. I am not liable for risks or issues associated with using or acting upon the information in this blog.
