What is mammographic density?
Mammographic density (also called breast density) is a term used to describe how breasts look on a mammogram. It measures and compares the different types of breast tissue that are seen on a mammogram. It cannot be seen or felt in a clinical examination by a doctor.
The types of tissue that make up the breast and can be seen on a mammogram are:
- fatty tissue
- glandular tissue (the milk ducts and lobules)
- connective tissue (which helps hold everything in place).
On a mammogram, fatty tissue appears as dark, while glandular and connective tissue appear white. Breasts that have a lot of glandular and connective tissue are described as dense. The expression ‘having dense breasts’ is used to refer to women who have high mammographic density.
Cancers also appear as white on a mammogram.
Why is mammographic density important?
Mammographic density is important because it can have serious implications for women who have dense breast tissue.
Later detection of cancers
The glandular and connective breast tissue can hide, or mask, cancers, making early detection difficult. This is because cancers, glandular tissue and connective tissue all appear as white on a mammogram – cancers can be hidden by the healthy tissue.
Women with dense breasts are at increased risk of having cancers missed on a mammogram and therefore at increased risk of having an ‘interval’ cancer. Interval cancers are cancers that are found between scheduled screening mammograms. They are usually diagnosed after a woman presents to her doctor with symptoms (e.g. a lump, nipple discharge). Because they are detected later, interval cancers may require more treatment and may have poorer outcomes.
The Information Forum on Mammographic Density, INFORMD, has developed a video that helps to explain mammographic density and why it is important. You can view it on YouTube.
Risk factor for breast cancer
Having dense breasts is a risk factor for developing breast cancer. Mammographic density should be considered with a woman’s other breast cancer risk factors to estimate her risk of developing breast cancer and to advise on screening and management. Read more on this in the section below titled Breast cancer risk.
Is breast size related to the mammographic density?
Breast size does not relate to how dense your breast tissue appears on a mammogram. Density depends on how much fat your breast has compared with how much glandular/connective tissue.
How is mammographic density classified?
The level of mammographic density can be scored by radiologists (specialists who read mammograms) along a scale from very low density (mostly fatty tissue) to mostly dense (mostly glandular/connective tissue). One of the most used scales is the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS), which radiologists use to classify mammographic density as A, B, C or D. Sometimes a numerical scale of 1, 2, 3 or 4 is used.
- Type A (almost entirely fatty, the lowest density): around 10% of women have mostly fatty, very low-density breasts.
- Type B (scattered areas of fibroglandular density): roughly 40% of women have this low level of density.
- Type C (heterogeneously dense): another 40% of women have this type of mammographic density, which is considered dense and may obscure small cancers.
- Type D (extremely dense): around 10% of women have extremely dense breasts, which lowers the sensitivity of mammography.
What factors influence mammographic density?
There are some factors that can influence density levels to some extent, for example young women and women with a lower body mass index (BMI) tend to have higher mammographic density.
Genetic factors are also important in determining the level of mammographic density. It is heritable, so women in the same family are likely to have the same density.
Other factors that can increase or decrease mammographic density to some extent include:
- having children (decreases)
- being on hormone replacement therapy (increases)
- going through menopause (decreases)
- being on tamoxifen (decreases).
All of these factors can change hormone levels in the body and influence mammographic density. However, while mammographic density usually declines with age, environmental influences or hormone levels, density is mostly determined when the breasts first form. This means some women will always sit in a high-density category compared with others in their age group, while others will always have lower density.
Breast cancer risk
As well as hiding or masking breast cancers, research shows that breast density is a risk factor for developing breast cancer.
Women with heterogeneously or extremely dense breasts (Types C and D on the BI-RADS scale) have a higher risk of developing breast cancer than women with fattier breasts.
Mammographic density should be considered with other risk factors, such as family history and the age of diagnosis of other family members, to help determine a woman’s risk of developing breast cancer.
It is important to note, however, that:
- Only about 10% of women have extremely dense breasts (Type 4 on the BI-RADS scale). Most women will have a level of mammographic density that is somewhere in the middle of the two extremes (neither mostly fatty nor extremely dense), where the level of risk from mammographic density is not as high.
- A woman’s mammographic density can vary at different points in her life.
- Many women who do not have dense breasts develop breast cancer. This is why it is important for all women aged 50 to 74 to have regular mammogram screening, regardless of their mammographic density.
- Many women with high breast density do not develop breast cancer.
Will BreastScreen tell me if I have dense breasts?
Unfortunately, most BreastScreen services do not advise women of their mammographic density. BreastScreen Western Australia is the only Australian screening program that notifies women if they have dense breasts.
In February 2022, BreastScreen South Australia is beginning a six-month research study into measuring and reporting individual breast density. The trial will run across three screening locations – Arndale, Hyde Park and a mobile screening unit visiting outer metropolitan Adelaide. Women attending these clinics will be provided with their breast density category and be invited to complete an online survey.
How can I find out if I have dense breasts?
Don't be afraid to ask your doctor about your mammographic density. Women have the right to know their breast density, their breast cancer risks and how sensitive a mammogram is at detecting cancers in dense breasts. They have the right to make informed decisions about breast cancer screening and their own health.
Some private screening clinics tell women about their breast density; however, you may need to pay to have a mammogram done privately. Talk to your GP if you think this is something you would like to do.
What screening is recommended for women with dense breasts?
Mammograms remain the recommended screening tool, despite the potential for some small cancers to be missed. Many cancers can be seen on a mammogram, even if you have dense breasts. There is currently not enough evidence to include additional screening modalities in the clinical guidelines. Clinical trials are being undertaken to look at adding other screening such as ultrasound and MRI.
However, anyone with very high mammographic density (Type D) and other risk factors should discuss supplementary screening options such as ultrasound or MRI with their doctor.
If you are concerned, you should talk to your GP or breast surgeon about what is best for you.
Tips to help if you have dense breasts
- Find out what screening options are best for you. Talk to your GP or breast surgeon about what is best for you.
- Speak to your GP about how you can reduce any lifestyle risk factors. Although you can’t change your breast density, there are a number of lifestyle changes you can make to reduce your risk of developing breast cancer and improve your overall wellbeing. These include maintaining a healthy weight, exercising regularly, reducing your alcohol intake and giving up smoking. Although these changes don’t guarantee you won’t develop breast cancer, they’ll give you a start towards reducing your risk. You can learn more about risk factors for developing early breast cancer in My Journey.
- Get to know your breasts and what is normal for you. Look in the mirror at your breasts and check your breasts from time to time. If you notice any changes such as a lump, nipple discharge or persistent new breast pain, please see your GP promptly - even if your last screening mammogram was normal.
More information about mammographic density is available at: