Effective June 1, 2015 Michigan law requires that mammography service providers inform patients if they have “dense breast tissue” on screening mammography. Breast density is the description of the mammographic appearance of the tissue within the breast. It describes the relative amounts of fibrous and glandular tissue which show up as white on the mammogram compared to fat, which shows up as black or gray on the mammogram. Some women have denser breast tissue than others for reasons that are not completely understood. For most women, breast density decreases with age. Yet, in some women, there is little change. Many women have dense breasts and it is not abnormal. Radiologists characterize each mammogram into one of four levels of overall density:
- 10% have extremely dense breasts
- 40% have heterogeneously dense breasts
- 40% have scattered areas of fibroglandular density
- 10% have almost entirely fatty breasts
All women who undergo mammography in Michigan will now receive a letter including information about their breast density. “Dense breasts” are considered to be in the higher two categories, either heterogeneously dense or extremely dense.
Greater density on a mammogram makes it more difficult to detect a breast cancer due to a masking effect. Dense tissue appears white but cancers appear white as well. The expanse of dense white tissue is more likely to obscure cancer then a fatty, less dense background. In addition, women who have higher breast density are at greater risk of developing breast cancer. This degree of risk is debated. A patient with extremely dense breasts is considered to have approximately twice (relative risk 2.1) the risk of breast cancer compared to a woman with average breast density. A woman with heterogeneously dense breasts has a 20% increased risk (relative risk 1.2) of breast cancer compared to a woman with average breast density.
Patients with dense breasts, together with their health care provider, will need to consider the possible need for supplemental screening. The three primary methods of supplemental imaging are:
- magnetic resonance imaging (MRI)
- whole breast ultrasound
- digital breast tomosynthesis (3-dimensional (3-D) mammogram)