BACKGROUND Solitary Fibrous Tumour (SFT) is an uncommon mesenchymal neoplasm of fibroblastic type arising anywhere in the body. Classic SFTs show neoplastic cells arranged in a pattern of less architecture with alternating hypo and hypercellular areas and prominent branching vasculature. Traditionally, CD-34 expression is the most consistent finding in SFTs, however this expression is also common in other tumours that may mimic SFT. Recently, a recurrent gene fusion NAB2-STAT6 has been identified as molecular hallmark of SFT. Molecular detection of the fusion gene and immunohistochemical expression of nuclear STAT6 can be helpful in diagnosing SFT. Aim-To study the expression of STAT6 in SFT. MATERIALS AND METHODS 23 cases of SFT diagnosed during a 2-year period. STAT6 immunohistochemistry was performed on 18 SFT cases and also on 8 possible mimickers of SFT, which were taken as the (negative) control. RESULTS Nuclear STAT6 positivity was present in 18/18 (100%) cases of SFT, which were usually diffuse (4+ in 16 cases, 3+ in 1 case and 1+ in 1 case and intensity was strong in 15 cases, moderate in 2 cases and weak in 1 case). All other tumour types tested were negative for STAT6. CD-34 was done in all cases at initial diagnosis. 16/18 cases (88%) were CD-34 positive. 2 cases (12%) which were CD-34 negative were STAT6 positive. CD-99 and Bcl-2 were positive in most of the cases. CONCLUSION Strong nuclear STAT6 expression is largely specific for SFT and is helpful in distinguishing it from histologic mimics.
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