Assessment of BI-RADS category 4 lesions detected with screening mammography and screening US: utility of MR imaging.

PURPOSE: To investigate the utility of magnetic resonance (MR) imaging according to different types of Breast Imaging Reporting and Data System (BI-RADS) category 4 findings from screening mammography and/or screening ultrasonography (US).
MATERIALS AND METHODS: This institutional review board-approved prospective study included 340 patients in whom 353 lesions were detected at screening mammography or US and were rated BI-RADS category 4 after appropriate conventional work-up. Written informed consent was obtained from all patients. Women underwent standard dynamic contrast material-enhanced MR imaging for further assessment. Women with negative or benign MR findings who did not proceed to biopsy underwent intensified follow-up for at least 18 months. Pure clustered microcalcifications were followed up for at least 24 months.
RESULTS: Of the 353 study findings, 66 (18.7%) were finally shown to be true-positive (23 cases of ductal carcinoma in situ [DCIS], 43 invasive cancers) and 287 (81.3%) were false-positive. Assessment of MR imaging findings led to a correct diagnosis of no breast cancer in 264 of the 287 false-positive findings (92%) and helped confirm the presence of breast cancer in 63 of 66 malignancies. The false-negative rate for pure clustered microcalcifications was 12% (three of 25 cases) because of three nonenhancing low-grade DCIS cases; in turn, MR imaging depicted additional invasive cancers in three women with false-positive findings from mammography and US. For mammographic findings other than pure clustered microcalcifications, MR imaging increased the positive predictive value (PPV) from 17.5% (21 of 120 cases; 95% confidence interval [CI]: 10.7%, 24.3%) to 78% (21 of 27 cases; 95% CI: 62.1%, 93.5%), with a false-negative rate of 0%. For all US findings, MR imaging increased the PPV from 12.9% (20 of 155 cases; 95% CI: 7.6%, 18.2%) to 69% (20 of 29 cases; 95% CI: 52.2%, 85.8%), again with a false-negative rate of 0%. MR imaging resulted in false-positive findings that led to MR imaging-guided biopsy in five of the 340 patients (1.5%).
CONCLUSION: MR imaging is useful for the noninvasive work-up of lesions classified as BI-RADS category 4 at mammography or US and can help avoid 92% of unnecessary biopsies. The false-negative rate was 0% for all US findings and for all mammographic findings except pure clustered microcalcifications. Additional invasive cancers were identified in three women with false-positive findings from mammography and US. © RSNA, 2014.