Daniel Kirsch, M.D.
Director, Breast Imaging and Intervention, Tower Imaging Medical Group, Santa Monica, CA
Director of Imaging, MargIe Petersen Breast Center at Saint John’s Health Center, Santa Monica, CA
Director, SheIla R. Veloz Breast Imaging Center, Henry Mayo Newhall Memorial Hospital, Valencia, CA
On April 1, 2013, California Senate Bill 1538 (Health Care—Mammograms) became law. The bill requires that facilities performing mammograms notify women if they have “dense breasts,” so they can consider discussing “further screening options” with their doctors. California became the fifth state to enact such a measure, following Connecticut, Texas, New York and Virginia. The bill was authored by Senator Jo Simitian (D-Palo Alto) after being suggested by Amy Colton of Soquel, California, who submitted the idea to the senator’s “There Ought To Be a Law” contest in 2011. Ms. Colton, apparently, had found out that she had “dense breasts” only after completing treatment for breast cancer, which her mammograms reportedly had failed to detect.
What does “dense breasts” mean, anyway?
The breast is composed of four elements: (1) the lobules (the actual glands), (2) the ducts (that carry the milk), (3) the adipose tissue (fat) and (4) the fibrous connective tissue (“scaffolding” which holds everything together).
Density is measured on mammography and is determined by the amount of water-containing structures (lobules, ducts and fibrous connective tissue) present relative to the amount of fat. The water-containing structures appear white, while the fat appears dark on mammograms. The more fibroglandular structures that are present in a breast and are seen on a mammogram, the higher the density.
Why is breast density important?
There are two primary concerns regarding the amount of fibroglandular tissue present on mammograms. The first is that mammograms of breasts that are dense are more limited than mammograms of breasts that are not dense (i.e., “fatty”). Simply put, it is more likely that a cancer present in a dense breast will be obscured and, therefore, not recognized, than a cancer present in a fatty breast.
The reason for this is that many cancers have the same x-ray attenuation as that of the dense, fibroglandular tissue. That is, both the cancer and the dense normal tissue appear white on mammography, and the margins of each may not be perceptible. In contrast, the white of the cancer may stand out against the dark appearance of the fatty tissue, making it much more likely to be identified. To identify cancer in a dense breast, one must rely on secondary signs that reflect changes in the tissue surrounding the cancer, such as spiculations or distortion of the architecture, which are often subtle, if visible at all.
This limitation of mammography in dense breasts is well known by radiologists that interpret mammograms and is noncontroversial. In contrast, ultrasound is not based upon these same x-ray attenuation principals, and, therefore, can detect some of these mammographically occult cancers. Breast MRI is even better at detecting them. This is one of the reasons why some have advocated “additional screening options” for women who have dense breasts. (There are other reasons why neither ultrasound nor MRI can actually replace mammography for screening at this time).
The second concern is the possibility that having dense breasts in and of itself confers a higher risk of developing breast cancer; in other words, that there is an actual relationship between breast density and developing breast cancer. This notion is currently a hot topic, particularly in the lay media, and appears to be encouraged by companies who manufacture software designed to quantitatively measure breast density on a mammogram. Careful review of the pertinent studies and the actual data paints a much more ambiguous picture. It turns out, that it is deceptively difficult to determine what, in fact, constitutes a “dense breast.” It does seem likely that there is a slightly increased risk of breast cancer in women whose breasts are subjectively considered to be dense. Most breast imagers believe that further study is necessary prior to incorporating breast density data into screening guidelines.
The decision of whether or not to pursue additional screening with breast ultrasound or MRI is complex. It should be remembered that mammography is the only modality that has actually been proven by studies to reduce mortality from breast cancer, and annual (or biannual) mammography after the age of 40 is recommended for screening by all medical organizations. Family history, results of any previous breast biopsies and other personal risk factors, as well as breast density, should all be taken into consideration when making this decision. This is a discussion that is best done with your breast care physician.
Gregory D. Pawelski