Borderline breast lesions and the limits of classification: insights from the <scp>UK</scp> National Breast Screening <scp>EQA</scp>  scheme

BACKGROUND: Previous concordance studies in breast pathology have demonstrated high levels of diagnostic agreement. This study aimed to characterize breast lesions associated with the lowest levels of concordance within the UK NHS Breast Screening Programme external quality assessment (EQA) scheme. METHODS: A total of 263 consecutive breast cases circulated between 2015 and 2025 were reviewed. Each case was assessed by up to 22 coordinators (expert reference panel assessors) and an average of 678 participants (all other pathologists enrolled in the EQA scheme). Cases with <80% concordance among coordinators were analysed in detail. RESULTS: Overall performance was high, with 67.4% of participants achieving 100% concordance across all circulated cases. Fifteen cases (5.7%) failed to meet the ≥80% concordance threshold. Mean diagnostic agreement in these cases was 57% (range 25%-79% among coordinators; 25%-87% among participants). Discordant cases predominantly involved atypical epithelial proliferations and papillary lesions, particularly those situated at diagnostic thresholds between established categories. Persistent challenges included distinguishing among lobular neoplasia subtypes, apocrine atypia, flat epithelial atypia, and related lesions, as well as between in situ and invasive papillary carcinoma. CONCLUSIONS: While overall concordance remains high, a small subset of biologically borderline lesions continues to generate significant diagnostic variability. These findings support further refinement of diagnostic criteria and targeted educational strategies to improve reproducibility in challenging breast lesions.