Background. Risk of progressive disease of gastrointestinal stromal tumors (GISTs) relies on mitotic index, size, and location of the tumor. However, manual mitotic counting on hematoxylin and eosin–stained slides (MMC-HE) is inefficient with low reproducibility. Manual count of phospho-histone H3 (MC-PHH3)-positive cells on immunohistochemical stained slides has been shown to have comparable reliability with MMC-HE. This study aims to confirm the reliability of MC-PHH3 in GISTs compared with MMC-HE and then to further compare MC-PHH3 with computer-assisted image analysis of PHH3-positive cells (Comp-PHH3). Methods. The study included 119 patients with GISTs. PHH3 stains were performed. MC-PHH3 was assessed as counts/5 mm2 high-power fields. Whole slide images were captured and the tumor area with greatest mitotic activity was manually identified. The PHH3-positive cells were automatically counted in 0.5 mm2 using Ventana Virtuoso software. Results. MMC-HE ranged from 0 to 157/5 mm2. MC-PHH3 ranged from 0 to 35.6/5 mm2. Comp-PHH3 ranged from 0 to 66/0.5 mm2. Interclass correlation coefficient (ICC) indicates good agreement between the 3 pathologists for MC-PHH3 (ICC = 0.74, P = .42). There is a strong correlation between MMC-HE and MC-PHH3. The Spearman correlation coefficient was 0.63 (P < .0001). Lin’s concordance further indicated a moderate diagnostic agreement between MC-PHH3 and Comp-PHH3. Conclusion. MC-PHH3 is proposed as a superior alternative to MMC-HE with potential application in GIST reporting and prognostication. Furthermore, Comp-PHH3 may be a valid alternative to MC-PHH3.
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