Diagnostic interobserver variability of atypia assessment in columnar cell lesions among a group of expert breast pathologists in the United Kingdom and the Republic of Ireland, on behalf of the UK national coordinating committee for breast pathology.

Aims: Atypical ductal hyperplasia and flat epithelial atypia (FEA) have defined diagnostic criteria, yet there is variation in the interpretation of these criteria, particularly when the atypia is present in a background of columnar cell lesions (CCLs). This study focuses upon cases which are especially challenging or difficult to classify reproducibly according to existing criteria.

Methods and results: Thirteen breast pathology experts were asked to classify 10 challenging cases with CLLs as atypical or non-atypical. Interobserver agreement was calculated. After two consensus meetings, which explored the morphological features underlying the decision, the cases were reassessed. Finally, a photomontage was compiled as a visual aid for practising pathologists representing a range of straightforward cases and others where subjective interpretation causes disagreement within current diagnostic criteria. Overall interobserver agreement and pairwise pathologist agreement coefficients were both in the fair range (κ = 0.22 and κ = 0.3-0.4, respectively). This improved to moderate or substantial agreement (κ = 0.6-0.8) after two consensus meetings. The most controversial cases were atypical cases that lacked the regular rounded nuclei of FEA, and non-atypical cases that had florid architectural changes bordering on architectural atypia.

Conclusion: Among expert breast pathologists, interobserver agreement in the diagnosis of atypia in CCLs was higher in cases with classical features of FEA. Consensus was difficult to achieve if nuclear or architectural atypia fell outside the classical definition of FEA, suggesting that this category does not encompass the range of low-grade cytological atypia in CLLs. This study provides rationale for expanding the definition of atypia in CCLs other than FEA.