Abstract
Background: Nonmass lesions (NMLs) on breast ultrasound lack clear definition and encompass a broad range of benign and malignant entities. Given anticipated inclusion of NMLs in the BI-RADS 6th edition, thorough understanding of these lesions will be critical for optimal management. Objective: To evaluate interreader agreement for classification of lesions on breast ultrasound as NMLs and to identify imaging features associated with malignancy in these lesions. Methods: This retrospective study included 2007 patients (2005 female, 2 male; mean age, 54.0±9.6 years) who underwent ultrasound-guided biopsy of 2381 breast lesions between January 2020 and December 2020. Two radiologists independently classified lesions as masses or NMLs, using a definition of NMLs from a presentation at the 2023 Radiological Society of North America annual meeting. The radiologists attempted to reach consensus for discordant cases. Another radiologist recorded NMLs' mammographic and ultrasound characteristics. Pathological outcomes for NMLs were extracted from the EHR. Results: 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, 317/2381 (13.3%) of lesions were classified as NMLs, of which 101/317 (31.9%) had initial discordance. A total of 133/317 (42.0%) NMLs were malignant, including invasive ductal carcinoma (48/133), ductal carcinoma in situ (43/133), and microinvasive ductal carcinoma (34/133). A total of 30.8% of malignant NML lacked correlative mammographic abnormalities. Ultrasound findings with highest accuracy for identifying malignancy of NMLs were calcifications (65.6%), posterior shadowing (62.8%), and non-parallel 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=4.7) on mammography, and greater size (OR=1.03), non-parallel orientation (OR=8.7), and posterior shadowing (OR=6.3) on ultrasound. Conclusion: The analysis provides insights into reader variability for classifying ultrasound lesions as NML by an existing definition, as well as into imaging findings' potential utility for characterizing such lesions as malignant. Clinical Impact: These findings indicate the need for further precision and clarification regarding the definition of NMLs and for further investigation into which NMLs have greatest malignancy risk.
Published Version
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