Nonmass Lesions on Breast Ultrasound: Interreader Agreement and Associations With Malignancy.

Nonmass lesions (NMLs) on breast ultrasound lack clear definition and encompass a broad range of benign and malignant entities. Given the anticipated inclusion of NMLs in the BI-RADS 6th edition, a thorough understanding of these lesions will be critical for their optimal management. The purpose of the present study was to evaluate interreader agreement for classification of lesions observed on breast ultrasound as NMLs and to identify the imaging features associated with malignancy in these lesions. This retrospective study included 2007 patients (2005 women and two men; mean age, 54.0 ± 9.6 [SD] years) who underwent ultrasound-guided biopsy of 2381 breast lesions between January 2020 and December 2020. Two radiologists independently classified the lesions as masses or NMLs, using a definition of NMLs from a presentation at the Radiological Society of North America 2023 annual meeting. The radiologists attempted to reach consensus for discordant cases. Another radiologist recorded the mammographic and ultrasound characteristics of the NMLs. Pathologic outcomes for NMLs were extracted from the EHR. Interreader agreement for lesion classification (mass vs NML) was substantial (κ = 0.73) A total of 216 lesions were classified as NMLs by both readers independently; an additional 101 lesions were classified as NMLs by consensus review after initial discordance. Thus, 317 of 2381 lesions (13.3%) were classified as NMLs; initial reader discordance occurred for 101 of these 317 lesions (31.9%). A total of 133 of 317 NMLs (42.0%) were malignant, including invasive ductal carcinoma (48/133), ductal carcinoma in situ (43/133), and microinvasive ductal carcinoma (micro-IDC) (34/133). A total of 30.8% of malignant NMLs lacked correlative mammographic abnormalities. Ultrasound findings with the highest accuracy for identifying malignancy of NMLs were calcifications (65.6%), posterior shadowing (62.8%), and nonparallel orientation (59.3%). In multivariable analysis, variables showing significant independent associations with malignancy included calcifications (OR = 8.9), asymmetry (OR = 4.7), and mass (OR = 6.4) on mammography and greater size (OR = 1.03), nonparallel orientation (OR = 8.8), and posterior shadowing (OR = 6.3) on ultrasound. The analysis provides insights regarding reader variability for classifying ultrasound lesions as NMLs on the basis of an existing definition as well as regarding the potential utility of imaging findings for characterizing such lesions as malignant. These findings indicate the need for further precision and clarification regarding the definition of NMLs and for further investigation to determine which NMLs have the greatest malignancy risk.