Focal asymmetric densities seen at mammography: US and pathologic correlation.

The American College of Radiology (ACR) Breast Imaging Reporting and Data System (BI-RADS) defines four different types of asymmetric breast findings: asymmetric breast tissue, densities seen in one projection, architectural distortion, and focal asymmetric densities. These lesions are frequently encountered at screening and diagnostic mammography and are significant because they may indicate a neoplasm, especially if an associated palpable mass is present. Once these lesions are detected at standard mammography, supplementary breast imaging with additional mammographic views and ultrasonography (US) can be a key aspect of work-up. The role of US in this setting has not been clearly defined. However, a positive US finding such as a solid mass or an area of focal shadowing increases the level of suspicion for malignancy. A thorough knowledge of the patient's clinical history, along with a fundamental understanding of the ACR BI-RADS lexicon and the role and limitations of supplementary breast imaging, will allow more accurate interpretation of these potentially perplexing soft-tissue findings.