Ultrasound non-mass lesions of the breast: imaging features, pathologic risk, and interobserver agreement

PURPOSE This study aimed to evaluate the sonographic and clinical characteristics of breast non-mass lesions detected on ultrasound, identify factors associated with increasing pathological risk, and assess interobserver agreement among breast radiologists. METHODS This retrospective study included breast ultrasound examinations performed between January 1, 2021, and August 30, 2025. A keyword search in the picture archiving and communication system identified 2,002 examinations potentially describing non-mass findings. After exclusions, 666 potentially eligible lesions were independently evaluated by three breast radiologists. Lesions were classified as non-mass according to a predefined majority rule (≥ 2 of 3 observers) and assessed using a three-level clinical concern score. Final diagnoses were established by histopathology or ≥ 2 years of imaging follow-up. Associations between imaging and clinical variables and pathological categories were evaluated using univariable analyses and multivariable ordinal logistic regression. Interobserver agreement was assessed using kappa statistics and Gwet's AC1. RESULTS A total of 610 lesions in 610 patients were classified as non-mass by majority decision. Malignancy was identified in 13 of 610 lesions (2.1%). Interobserver agreement for non-mass classification was high, although agreement estimates differed between Gwet's AC1 (0.80) and Fleiss' kappa (0.24). A clinical concern score of 3 predicted malignancy with a sensitivity of 76.9% (10/13) and specificity of 98.6% (582/590). Multivariable analysis showed that architectural distortion [odds ratio (OR): 8.40; 95% confidence interval (CI): 3.89-18.16; P < 0.001], posterior acoustic shadowing (OR: 4.76; 95% CI: 1.84-12.28; P = 0.001), and vascularity (OR: 2.69; 95% CI: 1.22-5.97; P = 0.015) were independently associated with increasing pathological risk. CONCLUSION Most breast non-mass lesions detected on ultrasound are benign; however, architectural distortion, posterior acoustic shadowing, and vascularity are independently associated with increased pathological risk. CLINICAL SIGNIFICANCE Recognizing specific ultrasound features associated with higher pathological risk may help distinguish non-mass lesions requiring more intensive diagnostic evaluation from predominantly benign findings and support more consistent clinical management.