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Dense Breast Tissue: What It Means and What to Know

By BCRF | September 25, 2023

BCRF shares what it means to have dense breasts and how researchers are striving to improve screening for women who have them

When former Today co-host Katie Couric announced that she was undergoing treatment for breast cancer, she also revealed that she has dense breasts. Dense breasts are associated with an elevated risk of developing breast cancer, in part because dense breast tissue can make it difficult for doctors to see abnormal growths in mammograms that may be cancer. 

So, what does it mean to have dense breast tissue, and why is it important to know if you have it? BCRF dives into this topic and highlights research focused on improving breast cancer detection in women who have dense breasts. 

What is dense breast tissue?

Breasts are made up of different types of tissue: Fibrous or connective tissue that holds the breast in place; glandular tissue, which includes lobules and ducts that produce and transport milk; and fatty tissue that fills the space between fibrous and glandular tissue and helps give breasts their size and shape. Breast density is a measure of how much fibrous and glandular tissue (referred to as fibroglandular tissue) there is in the breast relative to fat tissue. If you have higher breast density, it means that your breasts are comprised of more fibroglandular tissue compared to fat.

Dense breasts are normal and occur in nearly half of women over the age of 40. On average, breast density is higher in women under the age of 40 and tends to decrease as women get older; 40 percent of women in their 50s and 25 percent of women aged 60 and over have dense breasts. Asian women are more likely to have dense breasts than women of other races. Studies investigating whether Black or Hispanic women have denser breasts than their white counterparts have yielded mixed results. 

According to the Centers for Disease Control, women are more likely to have dense breasts if they are pregnant or breastfeeding, take hormone replacement therapy, or have a lower body weight.

How do I know if I have dense breast tissue?

The determination is made by a radiologist, who reviews a mammogram and assesses the proportion of white, gray, and black areas on the image. Fibroglandular tissue appears white on a mammogram, so the more white there is, the denser the breasts. Dense breasts cannot be detected during a physical examination because breasts that feel firm to the touch do not necessarily indicate a higher breast density.

Starting in March 2023, the FDA began requiring mammography facilities to notify patients of their breast density status when providing mammogram results and to standardize the reporting. Previously, reporting standards varied from state to state, and several did not require notification.

Why is it important to know if you have dense breasts?

Breast density is just one factor associated with an elevated breast cancer risk. Researchers are still trying to understand the underlying links between dense breasts and cancer. Breast cancer is known to develop in glandular tissue, so having a greater amount of it may simply provide more opportunity for abnormal cells to grow.

Higher breast density can also make it more challenging for radiologists to see abnormal growths since they both appear white on a mammogram. Dense breast tissue can effectively “cloud” the mammogram and increase the likelihood that a potentially cancerous growth or tumor is missed.

BCRF investigator Dr. Graham Colditz is one breast cancer researcher who is interested in developing better methods to predict breast cancer in women with dense breasts. He and his team have shown that while breast density generally decreases with age, the rate of density decrease was significantly slower in a breast that was later diagnosed with cancer compared to breasts that remained cancer-free. They are currently collecting and analyzing mammogram data from 10,000 women over time in order to develop a model to predict cancer development. In a related study, Dr. Colditz and his team have also shown that having a family history of breast cancer was associated with having dense breasts in premenopausal women. 

How is dense breast tissue categorized?

Doctors use a system developed by the American College of Radiology (ACR) called Breast Imaging Reporting and Data System (BI-RADS) to interpret and report standardized mammogram findings. It is subjective in nature, so doctors are increasingly using automated software to assist in the classification process. BI-RADS classifies dense breasts into four categories:

Category A is for breasts composed entirely or almost entirely of fatty breast tissue. The mammogram from this breast will appear mostly dark gray or black and may have small amounts of dense tissue that appear light gray or white. About 10 percent of women fall into this category.

Category B is defined by scattered areas of dense tissue. A Category B mammogram will have both light and dark gray or black areas. Tumors can be obscured in areas of the breast that appear dense. About 40 percent of women fall into category B.

Category C is comprised of heterogeneously dense breasts. On a mammogram, these breasts will appear mostly light gray or white. About 40 percent of women fall into this category.

Category D is defined as extremely dense breasts. Most of the breast is made up of dense breast tissue, and the mammogram appears almost completely white. Ten percent of women fall into Category D.

Women in categories C and D are considered to have dense breasts.

How should dense breasts be screened for breast cancer?

Dense breasts make standard mammograms—also called 2D mammograms because they are a two-dimensional image of a three-dimensional breast—more challenging for radiologists to spot potential cancer. In addition, many women receive 3D mammograms (also called tomosynthesis) as part of their standard screening, which has been shown to improve breast cancer detection in women with heterogeneously dense breasts, but results are mixed in women with extremely dense breasts. 3D mammography is like 2D mammography in that X-rays are used to create an image of the breast. However, it uses an electronic detector to capture multiple projections of the breast that are reconstructed by software to create several image slices. The images do not form a truly 3-dimensional image, but detailed image slices of the breast that are not obscured by overlapping tissue can be displayed and reviewed individually by a radiologist. 

Experts do not currently agree what secondary tests, if any, should be done across all women with dense breasts to screen for the disease. The following screening tests are most commonly recommended for women who have heterogeneously, or extremely dense breasts as determined by a radiologist and in consultation with her doctor. 

  • Whole breast ultrasounds (WBUS) use high-frequency sound waves inaudible to humans to create an image of the breast (also called a sonogram) and may be used as a supplemental screening tool after a 2D or 3D mammogram. There is no ionizing radiation used in ultrasounds and they rarely cause significant discomfort. Ultrasounds can differentiate between fluid-filled masses (unlikely to be cancer) and solid masses, which may require further testing to ensure they are not cancer.
  • Breast magnetic resonance imaging (MRI) is a noninvasive test that uses strong magnets and radio wave pulses to create an image of the breast. It requires intravenous injection of a gadolinium-based contrast agent to enhance the anatomy and blood flow in the breast. MRI is a sensitive test that can allow the radiologist to see lesions hidden within tissue that may be of concern more accurately than a mammogram or ultrasound. On the flip side, MRIs are often cost prohibitive.

These tests are more sensitive than standard mammography, but they may indicate areas of concern that are not cancer or benign, referred to as a false positive. Breast cancer researchers are actively working to develop new screening methods that detect cancer as early as possible while reducing false positive rates.

BCRF research to improve detection in women with dense breasts

To tackle this problem, BCRF investigators are testing new screening technologies.

Dr. Wendie Berg is comparing cancer detection and false positive rates in 3D mammography screening to contrast-enhanced mammography (CEM). CEM, also known as contrast-enhanced spectral mammography (CESM) or digital mammography (CEDM), is a relatively new technique that combines conventional mammography with a contrast dye containing iodine. Cancer cells attract more of the iodine-containing contrast agent compared to normal tissues, so it is easier to identify cancerous tissue in CEM than through standard 2D or 3D mammography.

Dr. Berg is currently enrolling patients to her Tomosynthesis or Contrast Enhanced Mammography (TOCEM) trial in which she is examining rates of cancer detection in women who screened annually via 3D mammography and CEM for a period of four years. She and her team anticipate that cancer detection rates using CEM will be similar to rates from MRI screening but with fewer false positives. Additional benefits of using CEM over MRI screening methods include lower cost and shorter examination times, and it is an alternative choice for patients with claustrophobia or metallic implants.

Dr. Christopher Comstock is also evaluating the use of CEM as a breast cancer screening option for women with dense breast tissue. He is the principal investigator for the Contrast Enhanced Mammography Imaging Screening Trial (CMIST), supported by BCRF, the ACR, and GE Healthcare. The trial began enrolling participants in spring 2023, and the analysis will help determine if CEM improves cancer detection and reduces false positives in women with dense breasts.

Other new screening techniques may be beneficial for women with dense breasts. An abbreviated breast MRI has been shown to have better invasive breast cancer detection rates compared with tomosynthesis among women with dense breasts. Abbreviated breast MRIs use the same technology as MRI but take fewer images over a shorter period, which means screening time is shorter and less costly. Molecular Breast Imaging (MBI), also known as scintimammography or breast-specific gamma imaging, uses radioactive material injected into the blood that accumulates more often in cancer cells than in normal cells. A gamma ray camera is used to detect the gamma rays emitted by the radioactive tracer. This technique does not provide the same anatomical information as a standard mammogram or ultrasound, but it allows doctors to visualize specific cells as they function within breast tissue—for example, rapidly dividing cells that may indicate cancer.  

One size does not fit all

There are many considerations when it comes to breast cancer screening and care. Having dense breasts is one of them. Ms. Couric’s experience is a good reminder that knowing if you have dense breasts is important when considering screening options. You can ask your doctor if you have dense breasts and, if so, discuss how you should be screened for breast cancer.