Mitotic Activity in Gastrointestinal Stromal Tumors: Can we use Phosphohistone H3 Immunohistochemistry Instead of Hematoxylin and Eosin for Mitotic Count?

Objectives: In gastrointestinal stromal tumors (GIST), malignancy potential is determined by the prognostic disease risk stratification based on mitosis, tumor size, and location. Phosphohistone H3 (PHH3) is an immunohistochemical marker showing mitotic activity in cells. In this study, we aimed to evaluate mitosis in GIST with PHH3, compare the results with hematoxylin and eosin (HE) stained slides, and examine its relationship with other prognostic data. Methods: Clinicopathological findings and survival were determined in GIST cases diagnosed between 2006 and 2017. The prognostic risk score was calculated according HE- and PHH3-based mitosis. The cases were classified as Group I: HE + and PHH3 + and Group II: HE + and PHH3-. They were also grouped as those diagnosed before and after 2012 and the staining results of HE and PHH3 were re-analyzed. Results: Ninety-eight cases were included in the study. Mitosis was detected with both HE and PHH3 in 63.3% of the cases (62/98 cases) (Group I) while in 36.7% of cases, it was detected with HE but not with PHH3 (Group II). In only two cases, the risk score changed with PHH3 (very low -> intermedier grade). The ratio of HE + and PHH3 + cases in 2012 and after was significantly higher than HE + and PHH3 - cases. A statistically significant relation was found between HE- and PHH3-based risk scores (p<0.05). There was a significant difference between HE-based risk score groups in terms of survival (p<0.05), while no difference was observed between the PHH3-based risk score groups (p>0.05). Conclusion: In GIST cases, PHH3 can be used to determine mitosis in more recent blocks, taking into account the technical conditions of the laboratory, but it does not seem to be superior to mitosis detected by HE. Research should continue on new survival determinants for GIST.