This SFT arose in the nasal cavity. Note the respiratory epithelium overlying the tumor (left image).
As with SFTs in other locations, it is composed of bland spindled fibroblasts admixed with ropey collagen.
The vasculature may show staghorn branching (not very obvious in this case). The spindle cells form a vague patternless pattern.
Solitary fibrous tumor is a mesenchymal tumor that can arise in in the pleura or a variety of extrapleural locations including the peritoneum, retroperitoneum, mediastinum, upper respiratory tract, orbit and soft tissues.
They are composed of bland, spindled CD34-positive fibroblasts in a "patternless" pattern with interspersed ropey keloidal collagen. The vasculature may demonstrate a staghorn, hemiangiopericytous pattern. The IHC profile of SFTs includ positivity for CD34, bcl-2 and CD99. The neoplastic cells are negative for S100, desmin or actin (Fletcher).
These are polypoid lesions can be excised in the sinonasal tract.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 92-3.