Ki-67 proliferative index (Ki-67) is a predictive and prognostic factor in breast cancer (BC). However, some international committees do not recommend its use in routine practice due to insufficient clinical evidence and lack of standardisation and assessment method reproducibility. Scoring of Ki-67 by digital pathology may contribute to overcome these drawbacks. We evaluated 136 core biopsies of BC patients and calculated the correlation of Ki-67 scored by two breast pathologists with two methods, eyeballing visual assessment (EB) on the microscope and digital image analysis (DI), both assessed from hot spot areas (HS) and the average between hot and cold spot areas (AVE). Good and higher correlation between pathologists was observed for HS using DI in comparison to EB (0.861 vs. 0.828). Correlation in HS with both methods was very similar in homogeneous tumours (0.869 vs. 0.866). Lower correlation was found in heterogeneous tumours if EB was used instead of DI (0.691 vs. 0.838). Good agreement with DI in AVE areas was observed in both homogenous and heterogeneous tumours (0.898 and 0.887). Concordance of tumour molecular profiles based on Ki-67 was better using DI in comparison to EB (Kappa index, 0.589 vs. 0675). Whereas EB and DI were alike in homogeneous tumour, DI improved agreement in heterogeneous tumours, particularly in AVE areas. Subgroup analysis for tumour grades also showed improvement of correlation by DI in AVE areas in all G1/G2/G3 groups. Digital pathology using AVE method can be useful for Ki-67 scoring in daily practice, especially in heterogeneous and G2 tumours, by a substantial improvement of agreement between observers and results accuracy.
Keywords: Breast cancer; Digital analysis; Immunohistochemistry; Ki-67; Prognosis; Reproducibility.