Nonmass Findings at Breast US: Definition, Classifications, and Differential Diagnosis.

A nonmass finding at US has been described as a discrete identifiable area of altered echotexture compared with that of the surrounding breast tissue that does not conform to a mass shape. Recognizing nonmass findings is important because breast cancer can manifest as such lesions, and US correlate findings for mammographic and breast MRI abnormalities may manifest as nonmass findings. The term nonmass finding is not part of the current Breast Imaging Reporting and Data System US terminology, and no standardized approach to classify and evaluate nonmass findings at US currently exists, despite the various classification systems proposed in the literature. There is also considerable overlap between the sonographic features of benign and malignant causes of nonmass findings. These limitations cause diagnostic difficulty in evaluating clinical significance and recommending appropriate management. The authors review the definitions and classification systems of US nonmass findings proposed in the literature and illustrate the sonographic features of nonmass findings to help radiologists identify them at US. A range of benign and malignant causes of nonmass findings are reviewed, and sonographic-histopathologic correlations of nonmass findings are discussed. Cases of breast MRI and mammographic findings that may manifest as US nonmass findings are presented. Radiologists can improve detection and interpretative accuracy, as well as correlation of mammographic and MRI breast lesions, by increasing their recognition and understanding of nonmass findings at US.© RSNA, 2020.