What is mammographic density?
Mammographic density (also called ‘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?
Mammographic density (MD) 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.
The level of MD can be scored or rated by radiologists along a scale from mostly fatty (very low density) to mostly dense 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 MD on mammograms along a scale of 1-4:
- Type 1 (almost entirely fatty, the lowest density): around 10 per cent of women who undergo a mammogram will have mostly fatty, very low density breasts.
- Type 2 (scattered areas of fibroglandular density): roughly 40 percent of women who undergo a mammogram will have this low level of density.
- Type 3 (heterogeneously dense): another 40 percent of women will have this type of mammographic density, which is considered dense.
- Type 4 (extremely dense): around 10 per cent of women undergoing a mammogram will have extremely dense breasts.
As this shows, there is a wide range of MD. Some breasts are mostly fat and some breasts are mostly breast and connective tissue (dense breast). But most women who undergo a mammogram (around 80 per cent) will be somewhere in the middle of these two extremes.
What causes mammographic density?
Mammographic density varies widely from one woman to the next but in each woman, density levels tend not to change much across their lifetime. This is because an individual woman’s level of mammographic density is thought to be mostly determined when the breasts first form, due to genetic factors.
Some factors can influence MD levels to some extent. Young women and women with a lower body mass index (BMI) tend to have higher MD. Other factors that can influence mammographic density to some extent include having children, being on hormone replacement therapy, and going through menopause. All of these factors can change hormones in your body and influence mammographic density to some degree. However, while a woman’s mammographic density can decline slightly with age, environmental influences or hormones, MD is mostly determined when the breasts first form, so some women will always have a high MD for their age or compared to the average, while other women will have lower MD.
How does mammographic 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 also shows that breast density can itself be a risk factor for developing breast cancer. For the approximately 40 per cent of women who have ‘heterogeneously dense breasts’ (Type 3 on the BI-RADS scale), the risk of developing breast cancer is thought to be 1.2 times greater than average. Women with extremely dense breasts (Type 4) are 2.1 times more likely than average to develop breast cancer, and up to 6 times more likely than women with mostly fatty breasts.
However, it is important to note that:
- Only a low proportion of women have extremely dense breasts (around 10 per cent of women undergoing a mammogram). Most women will have a level of MD 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, and can be scored differently from one mammogram or radiologist to another. This can make it difficult to draw conclusions about a woman’s individual level of risk based on mammographic density alone.
- Even in women with extremely dense breasts, the risk of developing breast cancer is still not as high as other well-known risk 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 can 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.
How do health professionals use breast density?
At the time, most Australian breast screening centres do not routinely measure or record mammographic density. Mammographic density is also not routinely used in Australia to assess a woman’s risk of breast cancer. There are a number of reasons for this.
- Currently there is no standard practice in measuring mammographic density and predicting risk (either the risk of a cancer being masked, or the risk of developing breast cancer in the future). Although there are scores that radiologists use to rate mammographic density (such as BIRADS), these scores 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 mammographic density scores can vary so much, the commonly used scoring systems are not considered reliable enough to guide most diagnosis and treatment decisions.
- New technologies are becoming available which provide an automated measure of mammographic density (volumetric, automated tools such as Volpara and LIBRA). These technologies are considered more reliable because they are automated by a computer program and do not rely on visual estimates that may differ from one radiologist to another. However, these technologies are expensive and not yet widely available in Australia.
- While there is growing evidence that women with dense breasts have a greater overall risk of developing breast cancer, we currently do not have enough evidence about what can be done to reduce this risk, as there are strong genetic factors that determine MD. More research is needed in this area.
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. If you are interested in getting more information about your mammographic density, we encourage you to ask your screening centre or GP if they can advise you.
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 mammographic 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 screening options are best for you.
If you have dense breasts, it is also important that you speak to your GP about how you can reduce any lifestyle risk factors. There are a number of positive lifestyle changes you can make to reduce your risk of developing breast cancer and improve your overall wellbeing. These include maintaining a healthy weight, reducing your alcohol intake and quitting smoking. Although these changes provide no guarantee that you won’t develop breast cancer, they’ll give you a start towards reducing your risk. You can learn more by visiting our reducing your risk page. Or for more information, visit Cancer Australia’s webpage on what you can do to reduce your risk of breast cancer. You can also find additional information on Breasthealth.org's website.
Adapted with permission from Susan G. Komen® from www.komen.org, © 2017 Susan G. Komen® on 12/01/2017