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Breast density is not a measure of how breasts feel; it is the amount of glandular tissue in breasts that appears on a mammogram.

Breasts are made up of 2 main types of tissue – fatty and fibroglandular tissue. Fat appears dark on a screening mammogram while glandular tissue appears ‘dense’ or white – as does breast cancer.

High breast density can increase the risk of breast cancer and can reduce the visibility of breast cancers on a screening mammogram.

From 8 August 2023, BreastScreen SA will measure and report individual breast density for our clients as part of our regular breast cancer screening program. Clients will be provided with their individual breast density category in their results letter.

Dense breasts

These pictures illustrate the breast density categories as measured using the Breast Imaging Reporting and Data System (ACR BI-RADS® Atlas 5th Edition).

Why is breast density important?

Increased breast density is associated with an increased risk of breast cancer.

Increased breast density also lowers the accuracy or visibility of mammography for breast cancer detection due to a masking effect. As cancers are typically white in appearance on a mammogram, they may be camouflaged by dense breast tissue.

Higher interval cancer rates are also seen in women with higher breast density. Interval cancers, which are diagnosed within 24 months after a regular screening mammogram, are typically more advanced and symptomatic at the time of diagnosis and are associated with poorer outcomes.

How is breast density measured?

Breast density is a mammographic finding and has no association with the look and feel of the breast on clinical examination.

A woman’s breast density can only be measured from a mammogram. BreastScreen SA’s research uses a fully-automated software program, Volpara, to measure breast density. There is no additional radiation involved and no change to the screening experience for clients.

What are clients told about their breast density?

There are 4 categories of breast density, as measured using the Breast Imaging Reporting and Data System (BI-RADS), American College of Radiology (ACR) 5th edition.

Clients are told their breast density category as part of their regular screening results. Their nominated doctor receives the same information. The 4 categories are:

  • BI-RADS a (around 10% of women): Breasts are almost entirely fatty tissue.
  • BI-RADS b (around 40% of women): Breasts have scattered areas of fibroglandular tissue.
  • BI-RADS c (around 40% of women): Breasts are heterogeneously dense. The mix of non-dense and dense tissue may hide small cancers.
  • BI-RADS d (around 10% of women): Breasts are extremely dense, which can reduce visibility of cancers on mammograms.

Women with BI-RADS c or d classification are regarded as having high breast density.

Do all clients receive a breast density category?

For a small number of women (3-5%), we are unable to measure breast density due to physical restrictions during the screening appointment, an incomplete screen, or the software not generating an accurate reading.

In these circumstances, clients are not offered a repeat screening as additional exposure to radiation in deemed unnecessary, based on the ‘as low as reasonably achievable’ (ALARA) principle.

If a client has high breast density, will they need additional tests?

BreastScreen SA does not currently recommend additional tests for asymptomatic women with high breast density who have no other risk factors for breast cancer.

There are no randomised controlled trials that show supplemental imaging (such as MRI, ultrasound or tomosynthesis) saves additional lives in asymptomatic women with high breast density who have no other risk factors for breast cancer.

Supplemental imaging can be of benefit for women with high breast density who have other significant risk factors:

Women with BRCA 1 or 2 gene mutations are recommended to undergo annual breast MRI from the age of 30 to 50 years.

Women with a strong family history of breast cancer are recommended to undergo annual screening mammograms. Clients are eligible to screen at BreastScreen SA from the age of 40 years. Breast Screen SA’s definition of a strong family history is a:

  • a first degree relative diagnosed with breast cancer before the age of 50
  • a first-degree relative with cancer in both breasts (diagnosed at any age)
  • 2 or more first-degree relatives with breast cancer (diagnosed at any age)

Women with previous biopsy results showing a premalignant disease are recommended to undergo annual screening mammograms.

There is some early emerging evidence that supplemental imaging of women with high breast density may detect additional cancers. However, no trials have had sufficient follow-up to show evidence of improved health outcomes or survival benefits for women.

Given this, the incremental and relative benefits of additional testing need to be balanced against the potential harms to women, including heightened anxiety, false positive results (can be as high as 8.1% with ultrasound), unnecessary biopsies and over-diagnosis.

It is important to recognise that regardless of an individual’s breast density, mammography is still the best breast cancer screening test in a population-based screening program for asymptomatic women, aged 50 to 74 years.

What are the risk factors for breast cancer?

There are a number of lifestyle, environmental and genetic factors that influence a woman’s risk of developing breast cancer. These include increasing age, family history of breast cancer, HRT, BMI, alcohol intake, history of breastfeeding, number of childbirths, and the age at menarche and menopause.

Some risk factors are not modifiable and these need to be put into context so that women do not feel unnecessarily anxious or concerned. Many women have at least one risk factor but will never develop breast cancer, while others diagnosed with breast cancer may have had no known risk factors.

There are risk assessment tools available such as iPrevent.

What further advice can I provide for reducing breast cancer risk?

A healthy lifestyle can help to reduce women’s risk of developing breast cancer.

Women are advised to review their modifiable lifestyle risk factors with their GP, including alcohol consumption, diet, and levels of physical activity.

Where can I get more information?

To find out more, please phone 13 20 50 and ask to speak with one of our Medical Officers.