Phosphohistone H3 (PHH3) as a surrogate of mitotic figure count for grading in meningiomas: a comparison of PHH3 (S10) versus PHH3 (S28) antibodies.

Mitotic figure (MF) counting is important in the evaluation of meningioma grading. Nevertheless, mitosis assessment on hematoxylin and eosin (H&E)-stained slides may be problematic because of technical factors and pathologist's experience. Phosphohistone H3 (PHH3) is a mitosis-specific antibody that has proven to facilitate mitotic count in various tumors. However, the antibody performance between PHH3 serine10 (S10) and serine28 (S28) has never been compared in these tumors before. In this study, 48 cases of meningioma (28 grade I, 14 grade II, 6 grade III) were evaluated using immunohistochemical stains for four commercially available PHH3 (S10) and S28 antibodies to identify MFs and validate PHH3 intra- and interobserver reproducibility and agreement. Two pathologists counted MFs on both H&E- and PHH3-stained slides. H&E and PHH3 MFs were highly correlated (Spearman's rho = 0.96 for PHH3 (S10)-Biocare, 0.96 for PHH3 (S10)-CST, 0.91 for PHH3 (S28)-Abcam, and 0.89 for PHH3 (S28)-Santa Cruz. The mean difference between an H&E and PHH3 mitotic count is 0.81 for PHH3 (S10)-Biocare, 0.95 for PHH3 (S10)-CST, - 0.97 for PHH3 (S28)-Abcam, and - 0.97 for PHH3 (S28)-Santa Cruz. For comparison among four PHH3 antibodies, PHH3 mitotic counts had both a good intra- and interobserver reproducibility (p > 0.05). Regarding to World Health Organization (WHO) grade, there was not a significant discrepancy in the stratification of tumor grades for all four PHH3 antibodies in terms of interobserver agreement. The Cohen's kappa coefficient (K) was 0.93 for PHH3 (S10)-Biocare, 0.82 for PHH3 (S10)-CST, 0.76 for PHH3 (S28)-Abcam, and 0.80 for PHH3 (S28)-Santa Cruz. Considering survival analyses, all five proliferation indices were univariately associated with recurrences. Increased PHH3 mitotic indices (MIs) were significantly associated with recurrence-free survival in univariate Cox proportional hazards regression analysis (p < 0.001) and remained an independent predictor in multivariate analysis (p < 0.05). The appropriate prognostic cutoff values for recurrence prediction were 5 or more per 10 high-power fields (HPFs) for PHH3 (S10) and 3 or more per 10 HPFs for PHH3 (S28).