Mammographic breast density

Breast density (also called mammographic breast density) is a term used to describe how relative amounts of breast tissue appear on a mammogram. 

Breast density cannot be seen or felt on examination. On a mammogram, non-dense fatty tissue appears as dark areas, while glandular and connective tissues appear white. Most breast cancers also appear white on a mammogram, so breast density can affect the accuracy of a mammogram. Breasts that have a high proportion of glandular and connective tissue are whiter on a mammogram and described as ‘dense’. In general, breast density is higher in younger women and reduces with increasing age.

Increased breast density is associated with a higher risk of breast cancer, although the risk is less than having a first degree relative with breast cancer diagnosed before menopause (doubles the risk) or carrying the BRCA gene mutation (three to six times the risk).

Breast density has not routinely been reported in Australia, apart from BreastScreen WA (since 2008) and BreastScreen SA (since 2023). BreastScreen WA notifies women and their GPs if they have heterogeneously or extremely dense breasts.

Breast density classification

Radiologists can estimate the level of breast density using the American College of Radiology’s Breast Imaging Reporting and Data System (BI-RADS). 

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 fibro-glandular tissue): approximately 40% of women have this low level of density
• Type C (heterogeneously dense): approximately 40% of women have this type of breast density that may obscure small breast cancers
• Type D (extremely dense): around 10% of women have extremely dense breasts that lowers the sensitivity of mammograms due to the masking of some mammography features that identify cancer

Breast density can also be measured using computer software to provide a percentage score and a BI-RADs score. BreastScreen WA notifies women if they have an empirically estimated breast density (BI-RADS Type C and Type D). Researchers at the University of WA found that over 70% of 5000+ woman surveyed said that knowing their breast density made them feel informed.

What influences breast density?

Breast density is complex and there are a number of known influencing factors:

• Age – younger women have higher mammographic breast density because of a higher proportion of fibro-glandular tissue compared to fatty tissue. Breast density generally declines with increasing age.
• Genetic factors
• Exogenous hormones (increases)
• Being on tamoxifen (decreases)
• Having children (reduces)

Breast size does not relate to breast density.

A holistic approach

BreastScreen WA advises women notified with breast density to see their GPs for a consultation and clinical breast examination (CBE). This is an opportunity to discuss mammographic breast density in the context of overall breast cancer risk of an individual woman (family history, long-term alcohol consumption, age at menarche, number of childbirths and breast feeding), perform a CBE and discuss further management. 

There are no current randomised controlled trials to support routine supplemental breast imaging tests (e.g. ultrasound) in women with dense breasts. Research demonstrates that supplemental breast ultrasound in women with dense breasts increases cancer detection by 40% but has a high false positive biopsy rate (>90%). Tomosynthesis (3D mammography) has a high cancer detection rate but is not yet proven to show a survival benefit as a screening modality in women with dense breasts. 

Without studies demonstrating a survival benefit, unnecessary use of radiological investigations can lead to overdiagnosis and overtreatment. Therefore, it is important to have an individualised approach, which may result in some women, after a consultation and CBE, reasonably referred for supplemental breast imaging. For example, a breast lump was detected on CBE, and a breast ultrasound requested. 

Despite there being currently no clinical guidelines for breast screening or managing cancer risk for women with dense breasts, mammography provided by an audited, double-blind screening program such as BreastScreen WA still remains the best population screening modality for breast cancer in asymptomatic women aged 50-74 years.

Key messages

• Breast density is a complex topic requiring patient-centred care to provide information and decide on supplemental screening
• Breast density is a breast cancer risk factor and can reduce the sensitivity of mammograms to detect cancer
• Screening mammography is still beneficial irrespective of breast density score.

– References available on request

Author competing interests – nil