Mammographic density (also called ‘breast density’) is a term used to measure and compare the different types of breast tissue visible on a mammogram.
Breasts are made up of several different components that include fat, glandular tissue (the milk ducts and lobules) and connective tissue, which helps hold everything in place.
Breast tissue on a mammogram
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.
Glandular and connective tissue shows up white on a mammogram, fat shows up dark on a mammogram. Breasts are defined as ‘dense’ on a mammogram if the image is mostly white
Classification of mammographic density
Mammographic density (MD) only describes how breasts look on a mammogram. It isn’t a measure of how the breasts look or feel, and it cannot be determined in a clinical examination 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 mammographic density can be scored or rated by radiologists along a scale from mostly fatty (very low density) to mostly dense. 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 as A, B, C or D. Sometimes a numerical scale of 1, 2, 3 or 4 is used instead.
- Type A (almost entirely fatty, the lowest density): around 10 percent of women who undergo a mammogram will have mostly fatty, very low-density breasts.
- Type B (scattered areas of fibroglandular density): roughly 40 percent of women who undergo a mammogram will have this low level of density.
- Type C (heterogeneously dense): another 40 percent of women will have this type of mammographic density, which is considered dense and may obscure small masses.
- Type D (extremely dense): around 10 percent of women undergoing a mammogram will have extremely dense breasts, which lowers the sensitivity of mammography.
As this shows, there is a wide range of mammographic density. Some breasts are mostly fat and some breasts are mostly glandular and connective tissue (dense breast).
Most women who have a mammogram (around 80 percent) will be somewhere in the middle of these two extremes.
Factors that influence mammographic density
Mammographic density varies widely from one woman to the next but in each woman, density levels tend to not change much across their lifetime. This is because, due to genetic and environmental factors, an individual woman’s level of mammographic density is thought to be mostly determined when the breasts first form.
Some factors can influence mammographic density levels to some extent, for example, young women and women with a lower body mass index (BMI) tend to have higher mammographic density.
Other factors that can influence mammographic density to some extent include:
- having children
- being on hormone replacement therapy
- going through menopause.
All of these factors can change hormones in the body and influence mammographic density to some degree. However, while a woman’s mammographic density can decline with age, environmental influences or hormones, density is mostly determined when the breasts first form, so some women will always have a high density for their age or compared to the average, while other women will have the lower density.
Effects of mammographic density
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 on a mammogram. This means it is harder to pick up cancers on mammograms of dense breasts than mammograms of mostly fatty breasts.
Breast cancer risk
Research also shows that breast density can itself be a risk factor for developing breast cancer.
- For the approximately 40 percent of women who have ‘heterogeneously dense breasts’ (Type C 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 D) are 2.1 times more likely than average to develop breast cancer, and up to 6 times more likely than women with mostly fatty (Type A) breasts.
Conversely, those with low breast density (Type A) have a lower risk of breast cancer than average.
However, it is important to note that:
- Only a low proportion of women have extremely dense breasts (around 10 percent of women undergoing a mammogram). 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 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 percent 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.
What you can do if you are told you have dense breasts
- Find out what screening options are best for you. 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.
- Speak to your GP about how you can reduce any lifestyle risk factors. Most women with very dense breasts will not go on to develop breast cancer. When considering a woman’s individual risk of developing breast cancer, it is recommended that breast density be considered with other risk factors, such as age, family history, body mass index. Although you can’t change your breast density 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, exercising regularly, 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 (insert BCNA web link).
- Get to know your breasts and what is normal for you. Look in the mirror at your breasts and feel your breasts from time to time. If you notice any unusual changes in your breasts such as lumps, nipple discharge, or persistent new breast pain, even if your last screening mammogram was normal, please see your GP promptly.
Current BCNA action: Raising awareness of breast cancer risks in women with dense breasts
I wasn't informed that I had dense breast tissue and that a mammogram or ultrasound could therefore miss an invasive cancer. I was originally treated for a stage 0 grade 2 cancer. Further cancer in the contralateral breast was missed at further screenings- BCNA 2017 Member Survey participant
We need more effective screening for breast cancer in women with dense breasts. My cancer was missed and was not picked up until I noticed a palpable lump 3.5 cm diameter. Accordingly I had to endure aggressive treatment i.e. Chemo, radio and bilateral mastectomy- BCNA 2017 Member Survey participant
BCNA has heard the voices of our members many of whom are distressed at learning that they were not informed about their level of breast density and were therefore unaware that screening mammograms may miss not detect a new breast cancer.
We are calling on the following areas of reform for women with dense breasts:
- Extension of the WA BreastScreen program that advises all women with dense breasts that their screening mammogram is less sensitive to cancers and they should therefore be vigilant in looking for breast changes and speak to their GP if they would like further information on additional screening options such as breast ultrasound, tomosynthesis or breast MRI.
- Improved Medicare rebates for women with dense breasts who need additional screening options such as breast MRI in order to reduce out-of-pocket costs associated with breast cancer surveillance.
- Greater funding investment into automated programs that can better determine a woman’s levels of breast density to better assist her to make informed decisions based upon her individual risk.
- Development of clinical practice guidelines for the management of breast density so that women can be better informed of what their risk of breast cancer is and how they can best screen for breast cancer.
- Greater research into links between breast density and breast cancer risk.