— DEAR MAYO CLINIC: Every year after I have a mammogram, I am told that I have dense breasts. What does this mean? I have heard that a new test for women with dense breasts — MBI — might be better for me. What exactly is this? Would it be covered by insurance?
ANSWER: Mammogram screening plays a vital role in detecting breast cancer. But in women with dense breasts, it can be difficult to distinguish normal breast tissue from tumor tissue. It’s because of this that a team of scientists from Mayo Clinic developed a tool — molecular breast imaging (MBI) — for looking at dense breast tissue.
MBI isn’t a replacement for mammography, which remains the standard tool for screening for breast cancer regardless of breast density. However, MBI can be an important supplemental tool for finding tumors that are not visible on mammography because of the surrounding breast density.
Breasts are a mixture of fatty and dense tissue. Younger women tend to have more dense tissue, and older women have more fatty tissue. Mammography of breasts with more fatty tissue typically produces images in which the breast tissue appears fairly dark. In contrast, tumors generally appear white.
Dense breast tissue also looks white on a mammogram. Some describe viewing mammograms of dense tissue as being similar to looking through a frosted glass window. A tumor can easily hide in a dense tissue mammogram.
About half of women younger than 50 have breasts that are considered dense on mammogram images. The same problem is seen in one-third of women older than 50.
Most commonly, breast density is classified using a four-category system that’s based on the appearance of the breast tissue on a mammogram. To find out how dense your breasts are, ask for and read the details of your most recent mammography report. When the breast is 25 percent or less dense, the radiologist’s mammography report describes the breast pattern as “predominantly fatty.” The next category is described as “scattered fibroglandular densities,” followed by “heterogeneously dense” and finally “extremely dense.” Breasts are considered dense when they fall into these last two categories.