Mammogram vs. MRI: Breast Screenings

By Michael Lasalandra

Beth Israel Deaconess Medical Center Correspondent

A number of studies published in the last few years have left many women wondering if MRI technology might be better than mammography for the detection of early breast cancers.

After all, mammograms aren’t perfect: they can certainly miss some tumors. And Magnetic Resonance Imaging (MRI) is known to be more sensitive than mammography. It can find some tumors that mammography misses.

A case in point: a study in the New England Journal of Medicine last year looked at 969 women who had already been diagnosed with cancer in one breast, and found that in 3.1 percent of these women, MRI found cancer in the other breast that didn't show up yet on mammogram.

So shouldn’t women be asking their doctors for an MRI instead of, or at least in addition to, their regular mammograms?

The answer for most women is no, says Dr. Susan Troyan, director of the breast care program at Beth Israel Deaconess Medical Center.

“Women are certainly coming in asking about MRI, especially high-risk patients,” Troyan says. “There’s been a lot of press about it. But there are a lot of problems. After I explain it all to them, most women agree they can wait a year or two for this technology to improve a little bit.”

Some of the confusion centers around guidelines issued by the American Cancer Society in 2007. After evaluating many studies, a 15-doctor ACS panel concluded that all women with a high risk of breast cancer - 20 to 25 percent or greater in their lifetime - should receive an annual MRI in addition to mammography.

Women in this high-risk category include those with a strong family history of breast and/or ovarian cancer or those who are likely or known carriers of the BRCA1 or BRCA2 mutation (a breast and ovarian cancer gene mutation).

At the same time, ACS said women at moderately increased risk (15 to 20 percent lifetime risk) should talk with their doctors about the benefits and limitations of adding MRI screening to their yearly mammogram. Yearly MRI screening was not recommended for women whose lifetime risk of breast cancer is less than 15 percent.

Calculating these risks can be done using a formula that provides women with a number known as a Gail score.

But Troyan says unless one is definitely in the highest risk category, the exercise may be pointless, because of the many problems associated with breast MRI.

The biggest problem is that, because of its sensitivity, breast MRI is associated with a false positive rate of 20 percent or higher, says Troyan. Such false positives are subject to additional work-up, rescreening and, sometimes, biopsies, not to mention the additional costs and anxiety, she notes. The rates of false positives on mammograms are much lower, she adds.

At the same time, microcalcifications that can be markers for small invasive cancers or the earliest form of breast cancer, known as Ductal Carcinoma in Situ (DCIS), do not show up on MRI, she says.

Breast MRI also costs about 10 times more than mammography, she notes. “That’s really problematic when you are talking about large numbers of people,” Troyan says.

MRIs also take about an hour to perform, compared to just a few minutes for a mammogram, and they cannot be done on people who have metal in their bodies, such as pacemakers or defibrillators. On the positive side, they do not involve exposure to radiation.

Another problem is the fact that few hospitals now have dedicated breast MRI machines, so they have to use an attachment to their regular MRI machines, which makes scheduling breast MRIs difficult, according to Troyan.

At the same time, few hospitals have radiologists with much experience in reading breast MRIs. This contributes to the high false positive rate, she says. Even if such experienced radiologists were plentiful, a breast MRI done at one hospital often cannot be read at another hospital because there are so many different softwares and protocols in use, she adds.

But the main reason why it may be too early to jump on the breast MRI bandwagon is that there are no studies as of yet showing this technology would make a difference in terms of saving lives, even if it is able to detect tumors earlier, Troyan says.

“When we talk about different technologies or strategies for prevention or treatment, we like to see randomized trials with the endpoint of reducing the risk of death from breast cancer,” she says. “We don’t have those yet. This is a promising technology, but there is much to be done for it to reach the potential we hope for it.”

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