What role does breast density play?

Photo of gynecologist with patient (simonkr / iStock / Thinkstock)

The female breast is made up of glands, fat and connective tissue. Breast density is considered to be high if the breast has a lot of glands and connective tissue in it, but not much fat tissue. The risk of breast cancer can be influenced by breast density. And very dense breast tissue makes it more difficult to see breast cancer tumors in x-ray images (mammograms).

The proportions of glands, fat and connective tissue in the female breast don’t always stay the same. They change depending on various hormone levels. So a woman’s breast density can increase and decrease again during her menstrual cycle. The number of mammary glands decreases over time in many women, particularly during menopause, and then their breast density usually decreases too. But taking estrogen pills can increase breast density. Overweight women tend to have less dense breast tissue.

How is breast density measured?

Breast density can’t be determined by palpation (feeling). It also has nothing to do with how big or firm the breast is. The only way to measure breast density is with imaging techniques – usually an x-ray of the breast (mammography). Mammography is normally used to screen for breast cancer or take a closer look at a breast change.

If the breast density is high, the mammogram will show a lot of white areas (glands or connective tissue). Fat tissue, on the other hand, looks dark on the image. So a lot of dark areas means the breast density is low.

How are different levels of breast density described?

There are different levels of breast density. They are usually described using four categories:

  • Category 1: The breast mainly consists of fat tissue (very low breast density).
  • Category 2: A large proportion of the breast consists of fat tissue, and there are a few areas of dense glandular tissue and connective tissue.
  • Category 3: The breast has more glandular and connective tissue in it than fat tissue.
  • Category 4: The breast is almost completely made up of glandular and connective tissue (very high breast density).

Categories 3 and 4 are usually considered to be high breast density. But the measurement of breast density isn’t particularly accurate, and the outcome may vary depending on the doctor, when it was measured, the type of imaging technique, and the equipment used. Because of this, different specialists might categorize a woman’s breast density differently.

How many women have a high breast density?

High breast density is normal: About half of all women over the age of 40 have dense breast tissue. Most women have category 2 or 3:

Breast density Frequency in women over the age of 40 Frequency in post-menopausal women
Category 1 ~ 10 % ~ 10 %
Category 2 ~ 45 % ~ 60 %
Category 3 ~ 35 % ~ 27 %
Category 4 ~ 10 % ~ 3 %

What does having dense breast tissue mean for women?

Compared to women with a low breast density, women with dense breast tissue are somewhat more likely to get breast cancer. But according to current research, having dense breast tissue doesn't increase the risk of dying of breast cancer.

Breast density is just one of many risk factors. The biggest risk factors for breast cancer are your age and genes (family history). So your actual risk of getting breast cancer can only be estimated by taking all of the main risk factors into account.

Dense breast tissue – particularly category 4 – makes it harder for doctors to see breast cancer tumors in mammograms. This is because it might look just as white as a tumor on the x-ray image, and might hide tumors. If, on the other hand, the breast is mainly made up of "dark" fat tissue (low breast density), changes in breast tissue are easier to detect.

How high is the risk of breast cancer in women with dense breast tissue?

An average of 25 out of 1,000 fifty-year-old women will get breast cancer within the next ten years. The following table shows the influence of different levels of breast density on the risk of breast cancer:

Breast density

Out of 1,000 fifty-year-old women,

the following number will get breast cancer

within the next ten years:

Category 1 10
Category 2 18
Category 3 27
Category 4 32

These figures refer to fifty-year-old women who don't have any other significant risk factors.

In other words, the risk for women with category 3 breast density – which is most women with high breast density – is only slightly higher than the risk for all women. The increase in risk is greater for women with category 4 breast density. On the other hand, the risk for women with low breast density is significantly below the average.

How good is mammography at detecting breast cancer?

First off: Many tumors can also be detected in the mammogram images of women with dense breast tissue. While taking the images, the mammography equipment is adjusted to provide the best possible images in women with high breast density too. So if a woman decides to be screened for breast cancer, mammography is the first choice of examination. In Germany, all women between the ages of 50 and 69 are offered a free mammogram every two years.

Research has shown that breast cancer screening results are more reliable in women with low breast density than in women with high breast density: Fewer breast changes are overlooked in less dense tissue. The following table shows how many tumors are detected in breast cancer screening (mammography) – depending on the different levels of breast density:

Breast density How many tumors are detected in mammogram images?
Category 1 Almost 100 %
Category 2 ~ 83 %
Category 3 ~ 80 %
Category 4 ~ 50 %

There are two main reasons why tumors might not be detected in mammography screening. The first reason: The tumor develops in the two years between mammograms. The second reason: A tumor is actually not seen because the breast tissue is too dense.

Should women with high breast density have additional screening examinations?

Because some tumors might not be seen in a mammogram, some doctors advise women with dense breast tissue to also have an ultrasound examination of the breast or an MRI scan (magnetic resonance imaging). The idea is that any tumors that may not have been detected through mammography will be discovered through these examinations. But it isn't clear whether doing these additional examinations would have any benefits. Too many unanswered questions remain, so the examinations aren't a standard part of breast cancer screening. Although studies have shown that these additional examinations do in fact find tumors that weren't detected in mammograms, they are also more likely to find breast changes that aren't cancer. This can be stressful for women because they may be scared and have to have further examinations (biopsies) to find out what the abnormal tissue is.

Additional examinations can also lead to overdiagnosis. Overdiagnosis is the diagnosis and treatment of tumors that wouldn't have become noticeable in the person's lifetime if they hadn't been detected through examinations – for example because they wouldn't have grown bigger and caused problems. There is also no research showing that such additional examinations can fulfill the actual goal of screening: to lower the risk of dying of breast cancer.

For these reasons, women's breast density isn't usually checked as part of the German breast cancer screening program, and women generally aren't told how dense their breast tissue is. But, as with other examinations, you can get a copy of the x-ray images and ask another doctor (e.g. your gynecologist) to take a look at them.