Understanding Breast Density

Understanding breast density helps you make informed, confident decisions about screening.

breast density graphic
Source: https://www.acr.org/-/media/ACR/Files/Breast-Imaging-Resources/Breast-Density-bro_ACR_SBI.pdf

What is Breast Density?

Breast density is a common finding, but it plays an important role in how breast tissue appears on imaging and how easily changes can be detected. Dense tissue on its own doesn’t guarantee a problem, but it can make traditional mammograms harder to interpret and may influence which screening options are most helpful for you. Understanding your breast density gives you clearer insight into your breast health and helps you choose the most effective, personalized approach to ongoing screening and monitoring.

By law in Iowa, after you have a mammogram, you should be informed of your breast density.

breast density categories

Category A:
Almost Entirely Fatty

Considered very low density. This means there is little fibroglandular tissue present and the breast is mostly fatty tissue.

*Mammograms detect changes easily with minimal obstruction.*

Category B:
Scattered Areas of Fibroglandular Density

Considered low density. This means the fibroglandular tissue is dispersed throughout the breast among fatty tissue.

*Mammograms remain effective with only mild interference.*

Category C:
Heterogenously Dense

This means there are large areas of dense fibroglandular tissue, but still some fatty tissue.

*Dense areas obscure findings. Mammograms may miss abnormalities in dense areas.*

Category D:
Extremely Dense

This means the majority of the breast has fibroglandular tissue.

*This is the most concerning category as it makes tumor detection very difficult on mammography.*

Each of the four categories refers the the degree of glandular and fibrous tissue present as seen on the mammogram. Fatty tissue will appear dark on the mammogram, whereas fibrous and fatty tissue with be brighter and whiter in color. Tumors will also appear white in color.

Do dense breasts affect thermograms?

No! Fortunately, thermography offers a different view of the breast – a physiological one. This is unlike mammograms that look at the breast anatomically (looking for structures). A thermogram looks for the blood flow that is used to grow a tumor – a process called angiogenesis (formation of new blood vessels). All tumors need an increased blood supply in order to grow faster than the normal cells that surround it. This creates an area of inflammation visible on a thermogram that changes over time. If we have a baseline established for you, we can pick up those changes very early – regardless of your breast density. 

thermography image of right breast with thermal activity
Fibroadenoma with color Doppler Ultrasound

Do dense breasts affect ultrasounds?

No! Ultrasound works very differently from mammography. Dense breast tissue appears white on a mammogram (just like cancer), which makes interpretation harder. But ultrasound uses sound waves, not X‑ray density, so it can see through dense tissue without the “white‑on‑white” problem. That’s why it’s such a valuable supplemental tool for women with Category C or D density.

More Questions on Breast Density?

As mentioned above and as seen in the photo, dense breast tissue will appear a white color, compared to fatty tissue which is darker. Cancer is often white on a mammogram as well. Think of a pond: if the water is clear, its easy to see the fish swimming around; however, if the water is murky, it can be difficult to decipher what-is-what in the water. When you see a large clump of white tissue on a mammogram, this can lead to (1) missing of cancers that may be present in the dense tissue OR (2) additional unnecessary testing, biopsies, and stress for women. 

The American College of Radiology and other organizations still support the use of mammography if you have been told you have dense breast tissue. Many cancers can still be seen on a mammogram if you have dense breast tissue. It is important to know that mammograms are not your only screening choice and, in women with dense breast tissue, it can be best to take a multifaceted approach to screening. Thermography, MRI, and ultrasound are other ways to screen your breast tissue.  

The biggest risk with dense breast tissue is the fact that breast cancer can go missed on a mammogram due to the increased density. It can be difficult to find breast cancers when they are small when breast density interferes. The best way to lower your risk of missing breast cancer is to use a multifaceted approach to screening. This means using other screening tools to monitor your breast health – including thermography.  There is also some research that shows women with dense breast tissue may have a higher risk of breast cancer.

Laws are being passed in several states that require a density category be given to every woman that has a mammogram. In Iowa, the breast density law went into effect on November 15, 2017. This law mandates that after your mammogram, your results letter will have your breast density listed as one of the four categories listed above. If you are considered category C or category D, you will also receive a notice that states: you may be at increased risk of breast cancer due to your density AND that having dense breast tissue can interfere with your mammogram interpretation as cancer becomes more difficult to detect. It is VERY important that you read and understand this letter. All women with dense breast tissue should seek additional ways to monitor their breast health. 

The American college of radiology states that only 10% of women are a category D (extremely dense). However, only 10% of women are a category A (almost entirely fatty). This means 80% of women are either category B or category C. It is important to remember that category rankings are subjective – meaning, one radiologist may rate you as a B while another may rate you as a C. Most radiologists will agree on patients that are category A or category D. 

It is common for breast tissue to become more fatty as you age. This is why mammograms have better sensitivity in older women compared to younger women with denser tissue. Your breast density may change as you age or as your body changes (through diet, exercise, and other lifestyle choices). 

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