A case of dedifferentiated solitary fibrous tumor of the thoracic cavity.

Solitary fibrous tumors (SFTs), initially observed in the pleura, were later found to develop in almost any extrapleural site. Dedifferentiation within SFTs, a rare phenomenon, was characterized only recently, although it was previously described in soft tissue and bone tumors. We report a case of dedifferentiated SFT arising in the right pleura of a 69-year-old man. Computed tomography revealed a huge mass in the thoracic cavity. The tumor contained an area with a high degree of calcification and was heterogeneously enhanced. Macroscopically, the resected tumor was 175×145×135 mm in size. Morphologically and immunohistochemically, this was comprised of a typical SFT juxtaposed to a high-grade component including an osteosarcomatous component. These were sharply demarcated by thin fibrous septa. Furthermore, NAB2-STAT6 fusion transcripts were detected by reverse-transcriptase polymerase chain reaction in both conventional and high-grade components, supporting the concept of a dedifferentiation process in this tumor.