False Positives

False positives are a risk of any screening test. For mam-mography, Elmore et al.43 estimated that after 10 annual screening mammograms, nearly 24% of women had had a false-positive result at least once, with a cumulative risk of a false-positive mammogram of 49.1%. Of women who did not have breast cancer, 18.6% underwent biopsy after 10 mammograms.43 Short-interval follow-up of specific lesions, such as nonpalpable circumscribed masses and focal asymmetries has been validated,44-46 with risk of malignancy less than 2% among appropriately classified lesions. Importantly, the prognosis is not adversely affected by short-interval follow-up in this setting.

Biopsy of benign lesions seen only sonographically, and induced short-interval follow-up, are risks of screening ultrasound. Across the five series where specifics are detailed,27-30,32 after 38,602 screening sonograms, 1,137 (2.9%) resulted in biopsy and 134 (11.8%) biopsies showed malignancy. In the four series with details,27,29,30,32 short-interval follow-up was recommended in another 6.6% of women. Criteria for classifying a lesion seen only sonographically as probably benign have been proposed47,48 but require broader validation. Follow-up is generally performed only for nonpal-pable lesions, although one recent study suggests the combination of benign-appearing features on both mammography and sonography may allow follow-up of even palpable lesions49; further validation of such an approach is required. It should be noted that in all but one series30 only a single prevalence screen was performed: these rates of false positives are likely higher than would be seen on annual incidence screens.

With MRI, from 2% to 17% of women screened were recommended for biopsy based on MRI, and 24% to 89% of MRI-prompted biopsies proved malignant (see Table 28.2). Short-interval follow-up was recommended in 5% to 24% of women on the first screening round where specified38,39,50-52 and decreased to 3% to 7% when results of subsequent screening rounds were detailed.39,52 With MRI, the criteria for follow-up and risk of malignancy in lesions followed have not been widely studied. Liberman et al.51 report 7% of lesions seen only on MRI that were followed proved malignant, and another 3% of patients developed cancer elsewhere in their breasts during short-interval follow-up. It is encouraging that, in the series of Kriege et al.,38 275 of 4,169 (6.6%) of examinations were recommended for short-interval follow-up, with only 3 of 275 (1.1%) of those proving malignant. MRI-guided core and vacuum-assisted biopsy are becoming more widely available53,54 but require availability of scanner time and personnel. A facility that offers breast MRI should observe standardized technique and interpretive criteria55 and offer MRI-guided biopsy.

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