Observer agreement in sonographic glandular tissue component classification and sonographic breast density assessment: a multi-institutional study

The sonographic glandular tissue component (GTC) is a potential imaging biomarker for stratifying breast cancer risk in women with dense breasts. This study evaluated the reproducibility, reliability, and clinical applicability of GTC classification and GTC-based sonographic breast density assessment in a multicenter observer study. Fifteen readers (nine physicians and six sonographers) from 12 institutions independently assessed 100 paired ultrasound images twice, 1 month apart, with the dataset rearranged for the second reading. GTC was qualitatively classified into four categories according to its estimated proportion within the fibroglandular tissue (FGT): minimal, mild, moderate, and marked; minimal/mild were grouped as low GTC and moderate/marked as high GTC. Sonographic breast density was categorized as sonographic dense breast (high GTC), sonographic non-dense breast (visible fat lobules), or intermediate, with intermediate cases further divided into high or low categories using an FGT-to-FAT (subcutaneous and retromammary fat) ratio cutoff of 1. Interobserver and intraobserver agreement was evaluated using κ statistics. Substantial interobserver agreement was observed for the four-class GTC classification, the two-class GTC classification, and sonographic breast density assessment. Intraobserver agreement was moderate for the four-class GTC classification and substantial for the two-class GTC classification and sonographic breast density assessment. The majority-vote sonographic breast density assessment showed substantial agreement with the mammographic ground truth, whereas individual-reader assessments showed moderate concordance with mammographic breast density both overall and in intermediate-density breasts. No significant differences in agreement were observed between physicians and sonographers. Substantial interobserver agreement and moderate-to-substantial intraobserver agreement support the reproducibility and reliability of GTC classification and GTC-based sonographic breast density assessment.