Categorizing breast mammographic density: intra- and interobserver reproducibility of BI-RADS density categories

The inter- and intraobserver agreement (κ-statistic) in reporting according to Breast Imaging Reporting and Data System (BI-RADS®) breast density categories was tested in 12 dedicated breast radiologists reading a digitized set of 100 two-view mammograms. Average intraobserver agreement was substantial (κ=0.71, range 0.32-0.88) on a four-grade scale (D1/D2/D3/D4) and almost perfect (κ=0.81, range 0.62-1.00) on a two-grade scale (D1-2/D3-4). Average interobserver agreement was moderate (κ=0.54, range 0.02-0.77) on a four-grade scale and substantial (κ=0.71, range 0.31-0.88) on a two-grade scale. Major disagreement was found for intermediate categories (D2=0.25, D3=0.28). Categorization of breast density according to BI-RADS is feasible and consistency is good within readers and reasonable between readers. Interobserver inconsistency does occur, and checking the adoption of proper criteria through a proficiency test and appropriate training might be useful. As inconsistency is probably due to erroneous perception of classification criteria, standard sets of reference images should be made available for training.