Magnetic resonance imaging
Because of the relative insensitivity of mammography in women with inherited risk for breast cancer, a number of screening modalities have been proposed and investigated in high-risk women, including BRCA mutation carriers. Several studies have described the experience with breast MRI screening in women at risk for breast cancer, including descriptions of relatively large multi-institutional trials.[28,29,30,31,32,33,34,35,36] Several considerations must be kept in mind when reviewing these reports:
- The studies are variable in terms of the underlying population being studied, equipment and signal processing protocols, the manner of reporting results, and the manner in which sensitivity and specificity are calculated.
- The different screening tests (MRI and mammogram with or without ultrasound) are performed nearly simultaneously in these studies, and the screening modalities are compared to each other. Therefore, sensitivity is defined somewhat differently in these studies than in the American College of Radiology Breast Imaging Reporting and Data System (BI-RADS) of follow-up and outcome reporting.
- The number of screening rounds is limited, and the distinction between prevalent (first round) and incident cancer detection rates is often unclear.
Despite these caveats, the reported studies consistently demonstrate that breast MRI is more sensitive than either mammography or ultrasound for the detection of hereditary breast cancer. The results of five large studies are presented in Table 8, Summary of MRI Screening Studies in Women at Hereditary Risk for Breast Cancer.[28,30,31,34,37] Most cancers in these programs were screen detected with only 6% of cancers presenting in the interval between screenings. The sensitivity of MRI (as defined by the study methodology) ranged from 71% to 100%. Of the combined studies, 82% of the cancers were identified by MRI compared with 40% by mammography.
Concerns have been raised about the reduced specificity of MRI compared with other screening modalities. In one study, after the initial MRI screen, 16.5% of the patients were recalled for further evaluation, and 7.6% of subjects were recommended to undergo a short-interval follow-up examination at 6 months. These rates declined significantly during later screening rounds, with fewer than 10% of the subjects recalled for more detailed MRI and fewer than 3% recommended to have short interval follow-up. In a second study, Magnetic Resonance Imaging for Breast Screening (MARIBS), the recall rate for additional evaluation was 10.7% per year. The benign biopsy rates in the first study were 11% at first round, 6.6% at second round, and 4.7% at third round. In the MARIBS study, the aggregate surgical biopsy rate was 9 per 1,000 screening episodes, though this may underestimate the burden because follow-up ultrasounds, core-needle biopsies, and fine-needle aspirations have not been included in the numerator of the MARIBS calculation. The PPV of MRI has been calculated differently in the various series and fluctuates somewhat, depending on whether all abnormal examinations or only the examinations that result in a biopsy are counted in the denominator. Generally, the PPV of a recommendation for tissue sampling (as opposed to further investigation) is in the range of 50% in most series.