STAT6 immunohistochemical expression in meningeal-based tumors: Diagnostic of solitary fibrous tumor/hemangiopericytomas

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Background: Updated 2016 WHO classification of central nervous system tumors has unified solitary fibrous tumor (SFT) and hemangiopericytoma (HPC) as a single entity (SFT/HPC), Grading from I to III, in view of common genetic alteration namely fusion of NAB2 and STAT6 genes. Aim: This study aims to evaluate the role of STAT6 immunohistochemical expression in a cohort of meningeal-based tumors and its correlation with other pathological parameters. Materials and Methods: Sixty-two meningeal-based tumors were studied including 40 HPC/SFT (1 Grade I, 29 Grade II and 10 Grade III), 14 meningiomas (2 Grade I, 10 Grade II, 2 anaplastic meningiomas), three meningeal sarcoma, and five cases with features suspicious for SFT/HPC. The expression pattern of STAT6 and other histological findings were noted. Results: Nuclear or nuclear + cytoplasmic STAT6 positivity was seen in 40/40SFT/HPC (100%), diffuse in 30 and focal in 10 cases. Only 1/14 cases of meningioma (7.1%) showed weak nuclear positivity. All cases of meningiomas and 2/3 cases of meningeal sarcoma showed cytoplasmic positivity. Of the five suspicious HPC cases, four showed diffuse strong nuclear STAT6 positivity, based on which they were confirmed as HPC. Interestingly, of the 40 STAT6 positive SFT/HPC, five showed epithelial differentiation (AE1/AE3 and/or epithelial membrane antigen [EMA]). CD34 was negative in 4/36 SFT/HPC. CD34 was focally positive in 3/5 suspicious HPC cases, two of which were focally EMA positive as well, and both cases showed nuclear STAT6 positivity. BCl2 was negative in 5/27 SFT/HPC, all of which were nuclear STAT6 positive. Conclusions: Nuclear expression of STAT6 can serve as a highly sensitive and specific adjunctive diagnostic marker for meningeal-based SFT/HPC, especially in cases with nonconventional histology and immunohistochemical profile, where meningioma or high-grade meningeal sarcoma forms a diagnostic dilemma.