What Is Breast Density and Why Does It Matter?
1. What are dense breasts?
“Dense breasts” are words that describe the mix of fat, milk glands/milk ducts and fibrous tissue in a breast. The more glands and fibrous tissue a woman has (also called “fibroglandular” tissue), the “denser” her breast tissue.
Each woman has a unique mix of fatty and dense tissue in her breasts. Some women have very little dense tissue compared to fatty. Other women have more dense tissue compared to fatty. Most women’s breasts are somewhere in-between.
Are Dense Breasts Normal?
Dense breasts are normal and common. Dense breasts are not caused by illness or disease. But there are some reasons for concern. That is because dense breasts can make it harder for radiologists to find cancer on a mammogram. Also, dense breasts increase a woman’s risk for breast cancer.
Here is more to know:
- Dense tissue appears as light gray or white on a mammogram (see image on the right). In dense breasts, any lump (whether due to cancer, or not) can also look light gray or white. That makes it harder for radiologists to find cancers on a mammogram for women who have dense breasts. It is like trying to find a snowball in a blizzard.
- Dense breasts are a risk factor for breast cancer. A woman’s risk for breast cancer increases with the level of density in her breast.
Video: What is Breast Density?
2. What causes dense breast tissue?
Dense breasts are normal and are not caused by illness. All breasts are made of fat, and milk ducts/milk glands held together by fibrous tissue. The glands and fibrous tissue (or “fibroglandular” tissue) are referred to as “dense tissue”.
Each woman’s breasts are different and contain their own mix of fatty and dense tissue. Some women have breasts made of mostly fatty tissue. Some women have breasts with a lot of dense tissue. The more glands and fibrous tissue that a woman has, the “denser” her breast tissue is.
How can I tell if I have dense breasts?
The radiologist (the doctor who looks at your mammogram) can tell how dense your breasts are by how they look on your mammogram. You cannot tell if you have dense breasts by the way your breasts look or feel.
3. Women often ask how to diagnose dense breast tissue. “How is breast density on a mammogram determined?”
Radiologists are the doctors who determine whether a woman has dense breasts. They do so after looking closely at images from mammograms. Radiologists may also use computer software to measure breast density. Breast density cannot be determined by how a breast looks or feels.
There are 4 categories of breast density. Here is more to know about each category:
Category A. Fatty, or almost entirely fatty breasts. This means that most of the breast is made of fat tissue. On a mammogram, a fatty breast appears as mostly dark grey or black. While most of the breast is fatty tissue, there may be a small amount of dense (fibroglandular) tissue that looks light grey or white. About 10% of all women have fatty breasts. Breasts in Category A are not considered dense.
Category B. Scattered areas of fibroglandular density. The breast has a mix of fatty tissue which appears dark grey or black, and scattered dense (fibroglandular) tissue which looks light grey or white. It can be hard for radiologists to find cancer, which often also look light grey or white, in areas that are dense. About 40% of all women have this type of breast. Breasts in Category B are not considered dense.
Category C. Heterogeneously dense breasts. Large portions of the breast are made of dense (fibroglandular) tissue. On a mammogram, the breast appears mostly light grey or white. It is harder for radiologists to find cancer, which often also looks light grey or white. About 40% of all women have heterogeneously dense breasts. Breasts in Category C are considered dense.
Category D. Extremely dense breasts. Most of the breast is made of dense (fibroglandular) tissue. On a mammogram, the breast looks almost all white. This makes it very hard for radiologists to find cancer, which often also looks light grey or white. About 10% of all women have extremely dense breasts. Breasts in Category D are considered dense.
For more information, see Q+A “How do I find out my density category?”
4. Do you know your breast density? Are you dense?
In the U.S.A., most states now have laws that require breast density information to be included in the letter you receive after your mammogram. Even if you do not live in a state with a breast density inform law, you can ask your referring health care provider for this information. Your breast density is usually in the mammogram report sent to them. Click here to see what you must be told about breast density in your state.
In Europe, the facility that does your mammogram may either have this information or allow you to request it. Density Request forms are available for the UK here.
5. Why does breast density matter on my mammogram?
Cancers can be hidden or “masked” by dense tissue. On a mammogram, cancer is white. Normal dense tissue also appears white. If a cancer grows in an area of normal dense tissue, it can be hard or even impossible to see cancer on a mammogram.
It is like trying to see a snowball in a blizzard.
- If a cancer (white) develops in an area of fat (black or dark gray/grey), it is usually easy to see cancer even when it is small. The more fatty a breast is, the easier it is to see cancer on a mammogram.
- Dense breast tissue can hide cancers. The denser a breast is, the harder it is to see cancer on a mammogram.
Cancer on a Mammogram in a Fatty Breast vs. a Dense Breast
Learn more on this topic in our Patient Education Video series, Let’s Talk About Dense Breasts.
6. Do dense breasts affect my risk of getting breast cancer?
Yes. Dense breasts not only can hide cancer on a mammogram, but they also increase the risk of getting breast cancer. Cancers grow more often in dense tissue than in fatty tissue. The denser the breasts are, the higher the breast cancer risk. Women with the densest breasts (“extremely dense”) are 4 times more likely to get breast cancer than women with the least dense breasts (“fatty” breasts). Most women’s breasts have density in between these two categories (“scattered fibroglandular density” or “heterogeneously dense” breasts).
What are risk factors for breast cancer?
Most breast cancer occurs in women with no known risk factors other than being a woman and getting older. There are many risk factors that increase the chance of getting breast cancer. For example:
- Having dense breasts
- Having a family history of breast or ovarian cancer (particularly in your mother or sisters)
- Having an atypical breast biopsy (where breast cells from a small sample of your breast tissue were not normal, but not yet cancer)
Download, save or print this helpful Risk Checklist to discuss with your health care provider.
1. American Cancer Society. Breast Cancer Facts & Figures 2019-2020. Atlanta: American Cancer Society, Inc. 2019. Retrieved from https://www.cancer.org/content/dam/cancer-org/research/cancer-facts-and-statistics/breast-cancer-facts-and-figures/breast-cancer-facts-and-figures-2019-2020.pdf. Accessed April 20, 2020.
2. McCormack VA, dos Santos Silva I. Breast density and parenchymal patterns as markers of breast cancer risk: A meta-analysis. Cancer Epidemiol Biomarkers Prev 2006; 15:1159-1169
7. Should I worry about dense breast tissue? Are dense breasts abnormal?
Dense breasts are normal and common. Though normal, dense breasts do increase the risk of developing breast cancer and of having a cancer missed on a mammogram.
Many women have dense breasts. Density tends to decrease as a woman gets older.
Talk to your health care provider about whether you should have additional tests after your mammogram. Click here for 5 Facts Every Woman Should Know.