The Medical Minute: MRI mammograms are not for everyone

August 03, 2004

By John Messmer
Penn State Milton S. Hershey Medical Center

Last week's New England Journal of Medicine reported the results of a study on the use of MRI (magnetic resonance imaging) mammograms in women at high risk for breast cancer. The study showed that the MRI found more cancers than regular X-ray mammograms. So, you might ask if all women should now get MRI mammograms instead of regular ones. It might surprise you to hear that the answer is "no."

In this study, the women were all at high risk for breast cancer. They either carried the breast cancer genes or were statistically at higher risk based on family and personal history. The results can not be applied to women of average or low risk.

It seems counterintuitive not to use a more sensitive test for all women. The reason is due to how medical testing works: most medical tests, in particular imaging studies such as X-rays and MRIs, require interpretation rather than giving just a positive or negative answer. So first, the radiologist must interpret the image accurately.

The test itself has certain sensitivities, including the chance that the test will actually detect the disease and specificity, and the chance that when the disease is not present, the test will be negative. When performed on a person who is likely to have the disease, a positive result with a very sensitive test is most likely truly positive. If the test is very sensitive and is considered positive when performed on someone unlikely to have the disease, there's a good chance the test is wrong. If a test is not specific enough, some people who are determined to have negative results might actually have the disease. These values are used to calculate the positive predictive value, that is, the likelihood that someone with a positive test actually has the condition.

Because of the sensitivity of MRI mammography, more women who are likely to have the disease were diagnosed with MRI than with X-ray mammography. The problem is the specificity is lower, making the positive predictive value lower than regular mammography. This means that if MRI were done as the standard method of mammography in all women, many low-risk women would have falsely positive mammograms. With about 15 million mammograms performed annually and a cost for MRI about $600 more than regular mammogram, we as a nation would pay another $9 billion annually just for breast cancer screening. More importantly, many women would be subject to worry, unneeded biopsies and in some cases, unneeded mastectomy because of suspected cancer. The only women who would benefit would be the high-risk ones.

Because of the limited benefit to most women, MRI mammography is only being considered for high-risk women in whom traditional mammography might miss a cancer. The New England Journal study defined high-risk women as those with a cumulative lifetime risk of as least 15 percent. To calculate your lifetime risk, go to http://bcra.nci.nih.gov/brc/

High-risk women include those known to have the breast cancer genes; those with close relatives who have had breast cancer; women who have had no children or delivered their first child later than the early 20's; and those who have had breast biopsies, particularly if atypical tissue was in the specimens. Women who meet these criteria should discuss the possibility of MRI mammography with their doctors. Until this approach becomes standard, some insurance companies may refuse to pay for the more expensive test routinely. Your doctor should be able to help you decide which test is best for you.

(Media Contacts)

Last Updated March 20, 2009