A 35-year-old Japanese female visited us complaining of a soft subcutaneous tumor on her hip. Physical examination revealed a cystic lesion measuring 40×35 mm with a mosaic echo pattern and a rich blood flow. We surgically removed it. Histopathologically, the tumor showed a patternless architecture with branching hemangiopericytoma-like vessels. The tumor cells were immunoreactive for CD34 and STAT-6. This tumor had a NAB2-STAT6 gene fusion as determined by reverse transcription polymerase chain reaction. We diagnosed this case as a solitary fibrous tumor (SFT). SFTs are known as pleural mesenchymal neoplasms. Extrapleural SFTs are uncommon, but they may be found in various organs including the meninges, subcutaneous tissue, and soft tissue of the extremities. NAB2-STAT6 fusion genes are specific for SFTs and the detection of the fusion gene can be helpful for diagnosis. Recently, the use of immunohistochemistry for STAT-6 as a surrogate for detecting the fusion gene has been reported. SFTs are uncommon subcutaneous soft tissue tumors and should be differentiated from other spindle cell-type mesenchymal neoplasms by the detection of a NAB2-STAT6 fusion gene or immunohistochemical positivity for STAT-6.
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