Magnetic resonance imaging (MRI) can find breast cancers that mammograms miss in high-risk women, according to Dutch researchers. Their finding, published in the New England Journal of Medicine (Vol. 351, No. 5: 427-437), lends support to the notion that screening with both methods may be a better option for high-risk women than using either one alone.

"These new investigations expand our knowledge in a very important area, which is how to screen for breast cancer in very high-risk groups," said Robert Smith, PhD, director of cancer screening for the American Cancer Society. "The amount of information we have is quite thin with respect to what age to begin screening and the best way to screen in women who are at higher risk."

Annual screening with mammography is already recommended by the American Cancer Society and other organizations for women 40 and older at average risk of getting breast cancer. But for women at high risk -- those with a strong family history of the disease or with a genetic mutation that predisposes them to breast cancer -- a more aggressive schedule may be needed.

These women are at greater risk of developing breast cancer before age 40, when regular screening would ordinarily begin, or they may have cancers that grow very fast, developing between mammograms. Moreover, mammograms are less effective in younger women because their breast tissue is denser, making the images harder to read. And in some cases, the types of tumors high-risk women develop are less identifiable by mammography.

ACS guidelines advise high-risk women to discuss with their doctor other screening methods -- like MRI or ultrasound -- that can be used to supplement regular mammography. (Screening at a younger age and screening more frequently are also options.) MRI is a promising tool because it can find tumors even in very dense breasts, and can find very small tumors. So far, though, there is not enough evidence to recommend MRI or another screening tool instead of mammography for these women.

The Dutch group, led by researchers at Erasmus Medical Center in Rotterdam, was attempting to address that issue. They recruited 1,909 women at high risk for breast cancer because of a family history of the disease or a genetic mutation. The women were given a physical breast exam by a doctor every 6 months, and a mammogram and MRI scan every year.

Over the course of 4 years, 50 breast tumors were found. MRI found 32 tumors, of which 22 were not visible on the corresponding mammogram. Overall, mammography detected 18 tumors, of which 8 were not visible with MRI. Mammograms were better able to find cases of ductal carcinoma in situ (DCIS), a precursor to invasive breast cancer; 5 of 6 DCIS lesions were identified by mammogram. MRI found only 1 of 6 DCIS cases, but it was the one missed by mammography.

The tumors were generally smaller than those found in two similar groups of unscreened women who developed breast cancer.

"Our study shows that the screening program we used, especially MRI screening, can detect breast cancer at an early stage in women at risk for breast cancer," the researchers wrote.

The findings are not definitive enough to make explicit recommendations that high-risk women begin screening at a particular age and with a particular method or combination of methods, Smith said.

And they do not suggest that MRI is an appropriate screening tool for women at average risk of developing cancer.

For one thing, MRI is very expensive, running about $1,000 to $1,500, compared to $100 to $150 for a mammogram. And although MRI finds many cancers, it will not find them all, and it also finds many things that aren't cancer (a false-positive result), leading to unnecessary follow-up procedures.

"In our study, screening by MRI led to twice as many unneeded additional examinations as mammography (420 vs. 207) and three times as many unneeded biopsies (24 vs. 7)," the researchers wrote.

Those drawbacks make MRI impractical for use on women who aren't especially likely to develop breast cancer. For women at high risk, though, the trade-off is more balanced.

"The fact that the rate of false-positives is higher [with MRI] is of very little consequence to women at high risk," said Smith. "Everyone would like to avoid a false positive, but the greater priority is to detect breast cancer early."

Overall, Smith said, the study findings lend support to the ACS guideline for high-risk women to consider supplementing regular mammography with MRI.

"The current thinking is that mammography plus MRI offers greater advantages to younger, very high-risk women" than either method does by itself, he said. "The more we learn about which tumors MRI detects and which it does not detect, and how we account for failures in both modalities to detect breast cancer early, the greater the potential for establishing tailored [screening] regimens for high-risk groups."

Improving screening in this group of women could have important long-term implications, Smith noted. Many women who know they are at very high risk of developing breast cancer choose to have their breasts or ovaries removed to reduce the chances they will get cancer, a move known as prophylactic surgery. However, a higher risk does not mean that these women are certain to develop breast cancer. Better screening could allow some women to chose more intensive surveillance, or postpone the decision about surgery to a later time.

"The more we learn about early detection [in this group of women], the more we may gain confidence that more intensive, regular screening is a viable and perhaps even competing option to other, more difficult and nonreversible decisions such as prophylactic surgery," he said.