Clinical Question: Is magnetic resonance imaging (MRI) more accurate than mammography in women who are at high risk of developing breast cancer?
Setting: Outpatient (any)
Study Design: Cohort (prospective)
Synopsis: The authors identified 1,909 Dutch women 25 to 70 years of age who had at least a 15 percent or more lifetime risk of developing breast cancer. This group included 358 women carrying mutations, 1,052 with a 30 to 49 percent risk, and 499 with a 15 to 29 percent risk. Of this group, 4.6 percent were lost to follow-up, largely because they underwent prophylactic mastectomy. Another 4.7 percent refused MRI, usually because of claustrophobia. The mean age was 40 years, only 15 percent had never had breast cancer screening, and approximately 75 percent were premenopausal. The women underwent a clinical breast examination every six months, as well as annual MRI and mammography, which were interpreted independently by radiologists blinded to the results of the other study. The women were followed for a median of 2.9 years.
A Breast Imaging Reporting and Data System (BI-RADS) classification of zero (“need additional imaging”), 3, 4, or 5 was considered a positive screening test result (a classification of 1 or 2 indicates normal or nearly normal). The reference standard was histologic examination, and sensitivity and specificity were calculated by comparing the number of cancers detected by each method plus any interval cancers. In addition to sensitivity and specificity, the ability to detect earlier cancers was evaluated by comparison with an unscreened control group who had a similar risk of breast cancer.
MRI was more than twice as sensitive as mammography, regardless of the cutoff used to define an abnormal test result (79 versus 33 percent), although it was somewhat less specific (90 percent for BI-RADS of zero, 3, 4, or 5 as abnormal compared with 95 percent for mammography). The area under the receiver-operating characteristic curve, a measure of overall diagnostic accuracy, was significantly greater with MRI (0.83 versus 0.69). However, the lower specificity meant that women underwent more unnecessary additional follow-up examinations (420 versus 207) and had more unnecessary biopsies performed (24 versus seven) when MRI screening was added. The comparison with unscreened control groups, although imperfect, found that fewer women screened with both MRI and mammography were node-positive.
Bottom Line: MRI is better than mammography at ruling out breast cancer (i.e., more sensitive), but it is more likely to produce false-positive results (i.e., less specific). It also is more expensive and leads to more unnecessary biopsies and follow-up studies. This makes it inappropriate in women at low or average risk, but it may be a good option in women at high risk who understand the limitations. (Level of Evidence: 2b)