Can you find the cancer?
How a cancer would show in different density categories on a mammogram.
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. Please see Q+A #6 “Do dense breasts affect my risk of getting breast cancer?” here.
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.
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
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)
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
I have dense breasts and cancer might be missed on my mammogram, what else can I do?
For women with dense breasts, an additional breast screening test after a mammogram may find cancer not seen on their mammogram alone.
Talk with your doctor or health provider about which one of these tests is right for you.
Cancer Detection by Screening Tests
|If 1,000 Women with Dense Breasts |
are Screened With:
|Number of Women Found
to Have Cancer is About:
|2D mammogram alone||5|
|3D mammogram (tomosynthesis)||6|
|2D Mammogram PLUS|
Common Breast Screening Tests
|2D Mammogram PLUS|
Other Breast Screening Tests
|Molecular Breast Imaging (MBI)||13|
|Contrast-enhanced mammogram (CEM)||15
Rev. August 2022
There are several things to consider when making a choice about an additional screening test. Additional screening may find cancers missed on your mammogram. But, that testing may also find things that need even further testing – and that turn out not to be cancer (known as a false positive).
- Ultrasound is the most common additional test used after a mammogram. Ultrasound uses sound waves and does not involve radiation or an injection into your vein. Gentle pressure is applied to the breasts and rarely causes discomfort. An ultrasound screening takes about 15 to 20 minutes.
- Magnetic resonance imaging (MRI) involves injection of a gadolinium contrast solution into your vein that may feel a little cold while it is injected. You lie on your stomach and your breasts fit into two openings. The contrast-enhanced MRI is done in the tunnel of a large magnet. The magnet makes loud noises while generating images. Contrast-enhanced MRI does not use radiation and takes from 10 to 25 minutes.
- Molecular Breast Imaging (MBI) involves injection of a radioactive agent into your vein. About 5 minutes after the injection, each breast is placed between two detectors, similar to a mammogram but with less pressure. An MBI test takes at least 40 minutes. MBI is not yet widely available.
- Contrast-enhanced mammogram (CEM) requires injection of iodine-based contrast into your vein. This is the same contrast used in CT scans. It makes you feel warm all over and you may even feel like you might pee on yourself. After about 2½ minutes, you will have a mammogram. A CEM test takes about 10 minutes. CEM is not yet widely available.
For more information, see Q+A #26: “What are other breast screening tests after my mammogram?” here.