Diagnostic value of a monoclonal anti-STAT6 antibody for solitary fibrous tumors: immunohistochemical analyses

Solitary fibrous tumors (SFTs) have been shown to harbor the characteristic NAB2-STAT6 gene fusion. Some recent studies have suggested that STAT6 is a useful marker for the diagnosis of SFTs. The aim of this study was to evaluate the diagnostic value of monoclonal anti-STAT6 in the distinction of SFTs from histologic mimics. The expression of STAT6 was evaluated in tissue microarrays and in whole sections of 407 tumors (including 63 SFTs and 344 nonSFTs) using a monoclonal anti-STAT6 antibody (clone: YE361). Only STAT6 nuclear staining was recorded as positive. All cases of SFTs (100%) exhibited nuclear STAT6 expression, and 45 cases displayed 3+ and strong staining intensity (72%), 7 cases displayed 3+ and moderate staining intensity (11%), 4 displayed 2+ and strong staining intensity (6%), 3 displayed 2+ and moderate staining intensity (5%) and 4 displayed 3+ and weak staining intensity (6%). The vast majority of the nonSFTs (343/344, 99.7%) were negative for STAT6. Only 0.3% (1/344) of the nonSFTs (one case of synovial sarcoma) showed 1+ and weak staining intensity of nuclear STAT6 expression. The sensitivity and specificity of nuclear STAT6 staining were 100% and 99.7%, respectively. In conclusion, the monoclonal anti-STAT6 antibody used in this study is highly sensitive and specific for the differential diagnosis of SFTs. In small biopsy specimens, negativity for STAT6 cannot completely exclude the diagnosis of SFT. Similarly, focal positivity for STAT6 cannot completely exclude the possibility of nonSFTs.