Quantifying Ki67 for characterising molecular subtypes in breast cancer: a comparative study of visual assessment and digital image analysis.

AIMS: Ki67 index assessment is crucial for breast cancer classification and prognosis. We compared Ki67 by immunohistochemistry (IHC) using visual counting-eyeballing (VA-E), manual counting on images (VA-M) and digital image analysis (DIA) to evaluate agreement levels and the impact when using IHC as a surrogate for molecular classification. METHODS: The study included needle biopsies of invasive breast cancers. The Ki67 index was re-estimated by DIA and VA-M. The re-estimated values were compared with previously reported VA-E values. Agreement was assessed by the kappa (κ) coefficient, and Bland-Altman plots were used to analyse the mean difference in the index and its impact on molecular classification. RESULTS: The study included 389 invasive ductal carcinomas of no special type. The majority were grade 2 (n=205, 52.7%) and 44% (n=171) were hormone receptor-positive (HR+). Grade 1 and 2 tumours with a Ki67 index of <20% showed significantly lower estimates by VA-E than DIA. Bland-Altman analysis revealed mean differences of 1.8 (95% CI -0.24 to 3.4) for all IDC and 4.2 (95% CI -1.91 to 6.49) for HR+ luminal cancers. Re-evaluation of the Ki67 index by DIA reclassified 28/62 luminal-A-like tumours to luminal-B-like and 16/109 luminal-B-like tumours to luminal-A-like, with an absolute discordance of 25.7%. VA-M estimates correlated more closely with DIA than VA-E. CONCLUSIONS: Lower Ki67 was observed with VA-E compared with DIA. There was discordance in the classification of luminal-like tumours, highlighting potential clinical implications when applying Ki67 cut-offs. VA-M demonstrated greater concordance with DIA than VA-E. For facilities lacking DIA, VA-M may be performed for Ki67, particularly in low-grade HR+ tumours.