— 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.
MBI is designed to see beyond dense breast tissue. Instead of using low-energy X-ray, as in mammography, MBI relies on gamma radiation. This type of radiation has the advantage of being unaffected by breast tissue density.
Before the MBI images are made, a short-lived radioactive agent (radioisotope) is injected into an arm vein. The patient is then seated in front of the gamma camera, and the breast is positioned between two plates with light compression — only about one-third the pressure used in a mammogram. Two 10-minute images are taken of each breast. If breast tumor cells are present, they absorb this substance like a sponge and show up as hot spots on the resulting image.
Recent advances in the MBI gamma camera have made it possible to significantly reduce the radiation dose, making the reduced MBI radiation levels comparable to the dose that’s delivered during one to two digital screening mammograms.
Images generated from MBI provide physiological information about the breast similar to that of magnetic resonance imaging (MRI). And while MRI is radiation-free, can provide detailed images of the breast and is highly sensitive in detecting small breast cancers, the cost for this test can exceed thousands of dollars. MBI generally runs about $600. Although the MBI unit was approved by the Food and Drug Administration in 2010, most insurance companies don’t currently cover the cost of MBI as a screening test.
While not a substitute for mammography, MBI may aid in breast cancer detection in women with dense breasts. Although the tool isn’t yet widely available, it’s anticipated that this will change over the next few years. — Deborah Rhodes, M.D., Breast Diagnostic Clinic, and Amy Conners, M.D., Radiology, Mayo Clinic, Rochester, Minn.