Breast Density

Breast Screening Decisions does not incorporate information about breast density into your breast cancer risk assessment. This is because there is not yet data that allows us to accurately predict how breast density affects breast cancer risk in an individual woman. But many women have questions about breast density, and so we are providing that information to you here.

What is Breast Density?

Breast density is a radiologic assessment of how well x-rays pass through the breast tissue. It is a surrogate for how much of the breast is composed of glandular tissue and how much is fat. The radiologist reading the mammogram classifies the breast composition as one of the following -

  • Almost entirely fat (<25% glandular)
  • Scattered fibroglandular densities (25-50%)
  • Heterogeneously dense breast tissue (51-75% glandular)
  • Extremely dense (> 75% glandular)

Many states now have laws requiring that women be told if they have dense breasts on their mammogram. State laws requiring breast density notification define dense breasts as those that are heterogeneously dense or extremely dense.

Here is what you need to know to understand how breast density relates to breast cancer screening and breast cancer risk.

Breast density is subjective

Different radiologists may give the same mammogram different ratings. Use of computerized density measurement could reduce these differences, but there is not yet a standardized computer rating system.

Dense breasts are extremely common, especially in younger women

According to a recent report of mammograms performed in New York City, 74% of women in their 40s, 57% of women in their 50's, 44% of women in their 60's and 36% of women in their 70's have dense breasts.

Breast density can vary with the menstrual cycle and with age

Breast density is highest in the second half of the menstrual cycle, and usually deceases with age. This means that the same women being scanned at a different time of the month or at a later year may be classified into a higher or lower breast density category.

Increased breast density may be a risk factor for getting breast cancer

The mechanism is unknown, but it may be that breast density is just the end result of other factors that increase breast cell proliferation and activity - factors like genetics and postmenopausal hormone use.

If you compare older women in the two extreme categories of breast density, those with extremely dense breasts have a three to five-fold higher cancer risk than those with mostly fatty breasts.

Unfortunately, we have no way to translate individual breast density into individual risk. Researchers are trying to see if breast density can be incorporated into existing risk assessments such as the Gail Model, but at this point, breast density has not been shown to add much more than we already know about a woman's risk from using these models.

Recent research suggests that a single breast density reading may not be the best way to predict breast cancer risk, and that the increased risk associated with breast density may be confined to those women whose breast density does not decrease with age.

Dense breasts can obscure a cancer on mammogram

This makes mammogram less reliable in women with dense breasts. Digital mammograms may be better at finding breast cancers in women with dense breasts who are also peri-menopausal or younger than age 50, but it is not known if this translates into better outcomes.

Additional testing with ultrasound and MRI can find cancers that mammograms miss in women with dense breasts. Unfortunately, breast ultrasound and MRI screening tests are less specific than mammograms - which means that they are more likely to have false-positive results. Three times as many biopsies will be done, most of which will not be cancer.

Additional screening beyond mammograms adds significant costs to breast cancer screening

For some women, this additional cost may not be covered by insurance. For example, Connecticut has passed a law mandating that insurers cover additional ultrasounds, but New York has not.

We do not know if additional breast cancer screening beyond mammograms saves lives

Neither ultrasound nor MRI is currently recommended for women with dense breasts who are at low to average risk of breast cancer.

Additional screening beyond mammography is only recommended for women at greater-than-average risk of developing breast cancer - women with a known genetic mutation associated with breast cancer or a family history suggesting one of these mutations, women with a history of chest radiation, and women with a predicted lifetime risk of breast cancer >25% (based on the Gail Model or other breast cancer risk assessment model). Even in women at increased risk, the additional testing beyond mammograms can have many false-positive results.

What should you do if you've been told your breasts are dense?

If you are at increased risk of developing breast cancer due to personal or family history, you may want to consider adding ultrasound or MRI screening.

Otherwise, there is no recommendation that you do anything other than continue screening at whatever interval you and your doctor have decided is right for you. If you decide you want a breast ultrasound, it's important to understand the additional risk of a false-positive result that could require a biopsy, and that the additional screening has not been shown to decrease deaths from breast cancer in women at average risk.

More Information on Mammograms and Breast Density