Solitary fibrous tumor (SFT) is composed of spindled to ovoid cells in a patternless architecture with prominent stromal
collagen and
hemangiopericytoma-like vessels. Some
tumors show hypercellularity, nuclear atypia, and significant mitotic activity; the latter feature in particular often portends an aggressive
clinical course. SFT can sometimes be difficult to distinguish from other benign mesenchymal
tumors and
sarcomas. The most characteristic (albeit nonspecific) immunohistochemical finding in SFT is CD34 expression. A NAB2-STAT6 gene fusion, resulting in a chimeric
protein in which a repressor domain of NGFI-A
binding protein 2 (EGR1
binding protein 2) (NAB2) is replaced with a carboxy-terminal transactivation domain from
signal transducer and activator of transcription 6,
interleukin-4 induced (STAT6), was recently identified as a consistent finding in SFT. However, as these genes are located in close proximity on 12q13, this fusion can only rarely be detected by conventional chromosomal banding or fluorescence in situ hybridization analysis. Nuclear expression of the carboxy terminal part of STAT6 is a consistent finding in SFT of the meninges (so-called 'meningeal
hemangiopericytoma'). We investigated STAT6 expression by immunohistochemistry in SFTs and other soft tissue
tumors arising outside the central nervous system to validate the diagnostic utility of this novel marker. Whole-tissue sections of 231
tumors were evaluated, including 60 cases of SFT as well as other benign and malignant mesenchymal
neoplasms and sarcomatoid
mesotheliomas. Fifty-nine of 60 SFT cases (98%) showed nuclear expression of STAT6, which was usually diffuse and intense. All other
tumor types were negative for STAT6, except for three
dedifferentiated liposarcomas and one deep
fibrous histiocytoma, which showed weak staining. In conclusion, STAT6 is a highly sensitive and almost perfectly specific immunohistochemical marker for SFT and can be helpful to distinguish this
tumor type from histologic mimics.