Malignant lesions initially categorized as probably benign breast lesions: retrospective review of ultrasonographic, clinical and pathologic characteristics. 
The primary objective of this study was to review the ultrasonographic features of BI-RADS category 3 ("probably benign") lesions that eventually proved to be malignant.  A second objective was to investigate their clinical and pathologic features according to their palpability and time of biopsy.  Thirty-two (0.8%) of 4000 women with lesions that were initially classified as "probably benign" proved to be malignant and formed the study group.  The most common reason for a false negative assessment on ultrasound was a failure to recognize suspicious margin characteristics (28 of 32 malignancies, 87.5%).  Malignancy was more frequent in palpable (2.4%, 21 of 859) than nonpalpable lesions (0.4%, 11 of 3141, p < 0.001).  There was no statistical difference in the mean age, mean size of lesions, or tumor stage between patients who underwent early biopsy (n = 19) or biopsy after 6 months (n = 13).  Twenty-nine of the lesions (90.6%) were retrospectively recategorized as BI-RADS 4, which calls for early biopsy.  BI-RADS 3 lesions require especially careful assessment.