Analysis of Tumor Proliferation Markers in Early-Stage Luminal Breast Cancer: A Comprehensive Study Using Mitotic Activity Index, Ki-67, and Phosphohistone H3 Expression

<i>Introduction and Aim:</i> Routinely used proliferation markers such as mitotic activity index (MAI) and Ki-67 index show limited reproducibility due to high interobserver variability in breast cancer assessment. Phosphohistone H3 (PhH3), a novel proliferation marker, is gaining attention in breast cancer research. This study aimed to evaluate the inter-rater agreement among MAI, Ki-67, and PhH3 expressions in early-stage luminal breast cancer and assess the impact of replacing MAI with PhH3 index on tumor histological grading. <i>Materials and Methods:</i> Three pathologists assessed MAI, Ki-67, and PhH3 expressions in 66 early-stage luminal breast cancer specimens. Mitotic Activity Index was scored based on mitotic figures in an area of 2 mm² while Ki-67 index utilized a 14% threshold for positively stained nuclei. Phosphohistone H3 expression cutoff was set at 13 positive cells per 2 mm². The inter-rater agreement for the 3 variables was analyzed using Cohen kappa statistics. <i>Results:</i> Among the 3 parameters, the kappa score of the PhH3 expression reflected very strong agreement between the 3 observers (κ = 0.991, 0.907, and 0.916). Only moderate agreement was noted for MAI (κ = 0.898, 0.562, and 0.592) and substantial agreement for Ki-67 index (κ = 0.869, 0.673, and 0.678). Moreover, replacing MAI with PhH3 index led to upgrade of histological grade in 15% to 16% of patients. <i>Conclusion:</i> Our study demonstrated that PhH3 is a more reproducible proliferation marker than MAI and Ki-67. Incorporation of PhH3-based mitotic index in breast cancer grading might reduce the variation in the assessment of histological grade.