MRI detects missed breast cancers. (Radiology).

Women who carry a genetic mutation predisposing them to breast cancer should rely on magnetic resonance imaging (MRI) instead of mammography for their regular screenings, a new study suggests. But for women who don't harbor a mutation in either the BRCA1 or BRCA2 gene, the normal forms of which suppress cancer, the high cost of MRI may not be justified, says Christiane K. Kuhl of the University of Bonn in Germany.

Kuhl and her colleagues identified 462 healthy women who carry a BRCA mutation and tracked their breast health for 5 years. Such women face an 80 percent lifetime chance of developing breast cancer, compared with about 12 percent for women in general. During the study, 51 breast cancers developed in 45 of the women. MRI detected 49 of these cancers, whereas mammography revealed only 17, Kuhl reports.

In a separate study of 53 women with a BRCA mutation, MRI found some tumors that mammography missed and was better at revealing hard-to-detect precancerous lumps, says Mark E. Robson of Memorial Sloan-Kettering Cancer Center in New York. However, MRI also raised false alarms.

Doctors have used MRI to examine breasts for more than a decade, but the medical community has yet to agree on the technique's value in detecting cancer.

Because women harboring a BRCA mutation often have a family history of breast cancer, some receive frequent mammograms, which expose them to excessive radiation, Kuhl says. This could be particularly dangerous, she says, because a BRCA mutation may degrade these women's ability to repair radiation damage to DNA.

Robson balks at that. "It's unclear whether excess mammograms cause excess cancers," he says. While acknowledging that MRI has the ability to find hidden tumors, he remains hesitant to order it because of its cost and the risk of results indicating a tumor where there isn't one.--N.S.