Better see to better agree: Phosphohistone H3 increases interobserver agreement in mitotic count for meningioma grading and imposes new specific thresholds

Background: Mitotic count on hematoxylin and eosin (H&E)-stained slides is a crucial diagnostic criterion in meningioma grading. However, mitosis assessment on H&E slides can be impaired by technical factors and by pathologist's experience. Phosphohistone H3 (PHH3) serine-10 is a mitosis-specific antibody that has proven to facilitate mitotic count in various tumors. Methods: A series of 70 meningiomas (15 grade I, 40 grade II, 15 grade III) was used to validate PHH3 intra- and interobserver reproducibility and to identify PHH3-specific mitotic thresholds. Four pathologists with different experience in neuropathology counted mitoses on both H&E- and PHH3-stained slides. Results: H&E and PHH3 mitotic rates were highly correlated (Pearson's r = 0.92, P m = 0.83) and a good intraclass correlation (0.78), higher than H&E mitotic indices (R_m = 0.77, intraclass correlation = 0.71). After further stratification of meningiomas according to World Health Organization grade, PHH3 performed better in terms of interobserver concordance (Kendall's W = 0.761) compared with H&E (Kendall's W = 0.697). Referring to the same meningioma groups identified by World Health Organization grade as the gold standard, the volume under the receiver operator characteristic surface was 0.91, indicating a very good diagnostic ability of PHH3 scores in discriminating the 3 meningioma groups. The 2 optimal PHH3-specific cutoff values were 6.61 and 22.02. Conclusion PHH3 staining is a useful diagnostic complementary tool to standard H&E mitotic count, optimizing intra- and interobserver reproducibility. PHH3-specific mitotic thresholds should be adopted to avoid overgrading of meningioma when ancillary methods are employed.