Nonmass Lesions on Breast Ultrasound: Radiologic-Pathologic Correlation and a Practical Guide to Diagnostic Approach

Breast lesions that do not meet the criteria for a mass on ultrasound (US), analogous to asymmetry on mammography or nonmass enhancement on MRI, are frequently encountered during diagnostic or screening US. These lesions, referred to as nonmass lesions (NMLs), are discrete areas of altered echotexture compared to surrounding breast tissue, lacking the threedimensionality or conspicuity of a mass. Their subtle nature makes it difficult—particularly for less experienced US operators—to distinguish between benign and malignant NMLs. With increasing clinical recognition, the upcoming sixth edition of the American College of Radiology Breast Imaging Reporting and Data System may include NMLs as a distinct diagnostic category. This article illustrates the sonographic features of NMLs and their pathologic correlations, providing extensive representative examples across benign NMLs, benign NMLs with upgrade potential, and malignant NMLs. In addition, it offers a practical and structured guide for a diagnostic approach to aid clinical management.