Precise tiered tumor grading is essential for predicting prognosis, selecting different treatment options and for follow-up of brain tumor patients. Ki67 labeling index (LI) is widely employed in assessing aggressiveness of glial brain tumors. However, Ki67 is subject to interlaboratory variability, and its antigen is expressed on all cell cycle phases except G0, which hinders its usage as a precise criterion for assessing cell proliferation. Indeed, there exist peculiar observations pertinent to increases of Ki67 index in glial tumors following radiotherapy or treatment with tyrosine kinase inhibitors. Moreover, we have witnessed a reduced Ki67 labeling in a gemistocytic glioma during its rapid recurrence under temozolomide treatment. Therefore, we reviewed pitfalls in employing Ki67 indices for predicting glial tumor biology and tried to answer whether the mitotic marker PHH3 (phosphorylated histone H3) could provide additional information in predicting glial tumor biology. PHH3-based assessment of proliferating cell fraction provides novel potentials, but it has also its own weaknesses. It has not yet been determined whether it would be more advantageous to report: a mitotic count (MC) per unit-area (e.g., 10 high power fields (HPF)) or a mitotic index (MI) (per 1,000 tumor cells). Further, there exist peculiarities in terms of unexpectedly low or high PHH3 values in pilocytic astrocytomas and angiocentric gliomas, respectively. Indeed, we encountered almost no staining with PHH3 in our unique gemistocytic astrocytoma case. Hence, at least in some glial malignancies, PHH3 may not be necessary for cell proliferation. Awareness of the weaknesses of proliferation markers in brain tumors may improve patient monitoring and treatment.
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