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Breast density and mammographic screening

What is breast density?

Breast density is a term used to measure and compare the different types of breast tissue visible in a mammogram.

Breasts are made up of several tissues, including fat, glandular tissue (the milk ducts and lobules) and connective tissue, which helps hold everything in place. Glandular and connective tissue are denser than fat, and this difference shows up on a mammogram.

Breasts are defined as ‘dense’ on a mammogram if most of the tissue making up the breast is glandular/connective tissue, and there is little fatty tissue.

  • High breast density means there is a greater amount of glandular and connective tissue compared to fat.
  • Low breast density means there is a greater amount of fat compared to glandular and connective tissue.

How does dense breast tissue look on a mammogram?

Breast density only describes how breasts look on a mammogram. It isn’t a measure of how the breasts feel, and it cannot be determined in a clinical exam by a doctor. Although breast density varies from one woman to the next, younger women and women with smaller breasts are more likely to have dense breasts. For a lot of women, breasts become less dense after menopause.

There is a range of breast density. Some breasts are mostly fat (fatty breast) and some breasts are mostly breast and connective tissue (dense breast). The level of breast density can be scored or rated along a scale from mostly fatty to mostly dense tissue.

What causes breast density?

Breast density varies from one woman to the next, even in the same age group. Researchers think that an individual woman’s level of breast density is determined when the breasts first form, and this largely due to genetic factors. But breast density can change over time, when environmental factors and lifestyle may alter a woman’s breast density. These factors include age, body mass index (BMI), having children, being on hormone replacement therapy, and going through menopause. For example, factors such as having children reduces breast density, while taking hormone replacement therapy can increase breast density. This is largely due to hormonal changes that influence breast tissue.

How does breast density affect your risk of breast cancer?

Cancers can sometimes be hidden or ‘masked’ on a mammogram image if the breasts are very dense. On a mammogram, fatty tissue appears as dark, and glandular and connective breast tissue shows up as white, or ‘dense’. Cancers also show up as white, or bright, on a mammogram. This means it is harder to pick up cancers on mammograms of dense breasts than mammograms of mostly fatty breasts.

Research is beginning to show that breast density can be a risk factor for developing breast cancer. Women with very high breast density may be up to 5 times more likely to get breast cancer than women with very low breast density.

Breast density varies greatly by age and weight and over a woman’s lifetime. Dense breasts are more common in young women and women with a lower body weight index (BMI).

It is important to note that:

  • The number of women with extremely dense breasts, and so significantly greater breast cancer risk, is small. Around 7.4 per cent of women in the U.S. aged between 40 and 74 have extremely dense breasts (the highest density category). Most women will have a level of breast density that is somewhere in the middle of the two extremes (neither mostly fatty nor extremely dense). This means that for most women, it is more difficult to predict how breast density affects their breast cancer risk.
  • Even in women with very dense breasts, the risk is still not as high as other factors, such as age (75 per cent of new breast cancer diagnoses are in women aged 50 and over or having a BRCA1 or BRCA2 gene mutation.
  • 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 breast density.

How do health professionals use breast density?

At this time, most Australian breast screening centres do not routinely measure or record breast density. Breast density is also not routinely used in Australia to assess a woman’s risk of breast cancer. There are two reasons for this.

  • Currently there is no standard way of measuring breast density. Although there are scores for rating breast density, these scores all depend on how radiologists interpret what they see in the mammographic image. This means that women’s breast density scores can vary from one radiologist to another. Because breast density scores can vary, they are still not considered reliable enough to guide most diagnosis and treatment decisions.
  • While there is growing evidence that women with dense breasts have a greater risk of developing breast cancer, we currently have no evidence about what can be done to reduce this risk. More research is needed in this area. If you are interested in getting more information about your breast density, you may be able to ask your screening centre or GP if they can advise you.
  • Women are not routinely told about their breast density. However, in Western Australia, BreastScreen WA does provide women, and their GPs, with information on breast density if the radiologist assesses them as having dense breasts. While no score is given, BreastScreen WA advises women with dense breasts to see their GPs for regular breast examinations, and if women have signs or symptoms of possibly significant breast disease, GPs are advised to refer women for additional breast imaging.  

Screening for women with dense breasts

At this time, there are no special recommendations or screening guidelines in Australia for women with dense breasts. If a doctor has concerns about a cancer being masked on a mammographic image because of high breast density, they may recommend additional screening - such as a breast MRI or ultrasound. It is important to note that there may be an additional cost for this screening.

Currently, mammograms are still the only screening method proven to reduce the risk of death from breast cancer in women over the age of 45. However, alternative screening methods – including breast ultrasound, breast MRI, and breast tomosynthesis (each in combination with mammography) - are being studied to learn whether they improve detection in women with dense breasts compared to mammograms alone.

You also need to be mindful that BreastScreen Australia does not offer breast MRI and ultrasound screening. MRI and ultrasound are more costly, with no or limited Medicare rebates. This means there can be significant out of pocket costs.

What should you do if you are told you have dense breasts?

If you are told you have dense breasts by your screening clinic or doctor, you may want to talk with them about what other options are best for you.

Adapted with permission from Susan G. Komen® from www.komen.org, © 2017 Susan G. Komen® on 12/01/2017