FDG PET/CT and MR imaging of CD34-negative soft-tissue solitary fibrous tumor with NAB2-STAT6 fusion gene.

Extrapleural solitary fibrous tumor (SFT) is an uncommon mesenchymal neoplasm of intermediate biological potential. Herein, we describe the radiological, histological, immunohistochemical and molecular genetic features of an SFT arising in the left thigh of a 55-year-old woman. Magnetic resonance imaging exhibited a well-defined mass with intermediate signal intensity on T1-weighted sequences and heterogeneous high signal intensity on T2-weighted sequences. Contrast-enhanced T1-weighted sequences showed strong homogeneous enhancement of the mass. A prominent vascular pedicle was visible. Integrated positron-emission tomography (PET)/computed tomographic (CT) scan demonstrated a moderate 18F-fluorodeoxyglucose (FDG) uptake (maximum standardized uptake value, 4.45) in the mass. Following an open biopsy, wide excision of the tumor was performed. Histologically, the tumor was composed of a proliferation of spindle cells in a fibrous stroma with focal hyalinization. Thin-walled branching hemangiopericytoma-like vessels were observed. Immunohistochemically, the tumor cells were diffusely positive for signal transducer and activator of transcription 6 (STAT6) but negative for CD34. The MIB-1 labeling index was less than 5%. Subsequent reverse transcriptase-polymerase chain reaction analysis identified a nerve growth factor inducible-A binding protein 2-STAT6 gene fusion. Our case supports the utility of STAT6 immunohistochemistry as an adjunct in the diagnosis of soft-tissue SFT with loss of CD34 positivity. To the best of our knowledge, this is the first report showing the FDG PET/CT findings of soft-tissue SFT.