Breast density and screening

Breast density (also called mammographic density) is a term used to describe how breasts look on a mammogram. It measures and compares the different types of breast tissue. Breast density 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 areas, while glandular and connective tissues appear white. Cancers also appear as white on a mammogram.  

Breasts that have a lot of glandular and connective tissue are described as dense. The expression "having dense breasts" is used to refer to people who have high mammographic density. Men tend to have less glandular and connective tissue in the breast than women, so breast density does not generally pose an issue for men.

Is breast density important?  

Breast density is important because it can have serious implications for people who have dense breast tissue.  

Later detection of cancers  

The glandular and connective breast tissue can hide (or mask) cancers in a mammogram, making early detection difficult. This is because cancers, glandular tissue and connective tissue all appear as white on a mammogram. Thus, cancers can be “hidden” by the healthy tissue in a mammogram. 

People 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, etc). Because they are detected later, interval cancers may require more treatment and may have poorer outcomes.   

The Information Forum on Mammographic Density (INFORMD), has a video that helps explain breast density and why it is important.     

Risk factor for breast cancer  

Having dense breasts is a risk factor for developing breast cancer. Breast density should be considered with a person’s other breast cancer risk factors to estimate their risk of developing breast cancer and to advise on screening and management. Read more on this in the section "Breast cancer risk" below.  

Breast size vs 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.    

Breast density classification

The level of breast 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 commonly used scales is the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS), which radiologists use to classify breast density as A, B, C or D. Sometimes a numerical scale of 1, 2, 3 or 4 is used.  

The BI-RADS system defines breast density in the following categories: 

• 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. 

Influencing factors  

Several factors can influence breast density to some extent. For example, young women and those with a lower body mass index (BMI) tend to have higher mammographic density.

Genetic factors are also important in determining the level of breast density. It is heritable, so people in the same family are likely to have similar density.

Other factors that can increase or decrease breast density to some extent include:

• having children (decreases)
• being on hormone replacement therapy (increases)
• going through menopause (decreases)
• being on tamoxifen (decreases).

These factors can change hormone levels in the body and influence breast density. However, while breast density usually declines with age, environmental influences or hormone levels, density is mostly determined when the breasts first form.

This means some people will always sit in a high-density category compared with others in their age group, while others will always have lower breast density.

Breast cancer risk

Research has shown that higher breast density is not only a contributing factor in masking breast cancers in mammograms, but also a risk factor for developing breast cancer.

People with heterogeneously or extremely dense breasts (Types C and D on the BI-RADS scale, see above) have a higher risk of developing breast cancer than women with fattier breasts.

Breast 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 person’s risk of developing breast cancer.

It is important to note, however, that:

• Only about 10 per cent 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 person’s mammographic density can vary at different points in their life.
• Many people who do not have dense breasts develop breast cancer. This is why it is important for all women aged 50 to 74 years to have regular mammogram screening, regardless of their mammographic density. Screening mammograms are not recommended for men in Australia due to the low incidence of breast cancer in men.
• Many people with high breast density do not develop breast cancer.

BreastScreen and density

Most BreastScreen services do not advise people of their mammographic density. BreastScreen in Western Australia and South Australia are the only two states that currently notify people if they are identified as having dense breasts when they have their screening mammograms.

The people who are notified as having dense breasts are given follow-up information, including whether any further care or investigation is necessary. 

Do I have dense breasts?

Don't be afraid to ask your doctor about your breast density. People have the right to know their breast density, their breast cancer risks and how sensitive a mammogram is at detecting cancers in dense breasts.

People have the right to make informed decisions about breast cancer screening and their own health.

Some private screening clinics tell people 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.

Recommended screening

Mammograms remain the recommended screening tool to detect breast cancers, 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 methods such as ultrasound and MRI to enhance the detection of breast cancers.

However, anyone with very high breast density (Type D on the BI-RADS scale, see above) and other risk factors should discuss supplementary screening options such as ultrasound or MRI with their doctor.

Tips for you

• Find out what screening options are best for you. Talk to your GP or breast surgeon for advice.  
• 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 choices you can make to reduce your risk of developing breast cancer and improve your overall wellbeing. Although these changes don’t guarantee you won’t develop breast cancer, they’ll give you a start towards reducing your risk.  These include:
  • maintaining a healthy weight
  • exercising regularly
  • reducing your alcohol intake
  • giving up smoking.

• Get to know your breasts and what is normal for you. Look in the mirror at your breasts, and check your breasts regularly. 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.