Answer: Solitary fibrous tumor

Histology: The lesion consists of localized mass arising from the dura. The cells are uniform and spindle without significant atypia. Mitoses are not identified. Areas of fibrosis and hyalinization, with bundles of collagen, can be seen. Focal myxoid areas can also be seen. The tumor is very vascular, with occasional branching vessels seen.

Discussion: Solitary fibrous tumors (SFT) may occur in any location, however the usual presentation is that of a circumscribed pleural mass. The origin of this tumor is still debated. SFT are characterized by bland spindle cells in a fibrous background with bundles of dense collagen. Areas of the tumor may be less cellular and myxoid. In contrast to sarcomas, SFT typically do not show significant atypia or abundant mitoses. SFT with increased cellularity, pleomorphism and more than 4 mitoses/10 HPF are regarded as malignant variants.

The main differential diagnosis is from hemangiopericytoma and fibrous meningioma. Both SFT and hemangiopericytoma can show high cellularity with oval plump cells and show branching vascular channels within the tumor. In contrast to hemangiopericytoma, SFT are characterized by prominent eosinophilic collagen bands. Unlike SFT, fibrous meningioma contain glycogen and show a storiform pattern, psammoma bodies and calcification. Immunohistochemical stains are helpful in differentiating the tumor from other histologically similar neoplasms. SFT are negative for EMA and cytokeratins, but are positive for CD 34 and BCL-2 in a diffuse and strongly positive pattern. Fibrous meningioma show EMA and S100 staining. Although hemangiopericytoma can stain with CD 34, it often shows weak and patchy staining in contrast to the diffuse and strong pattern seen with SFT. In addition, unlike SFT, hemangiopericytoma show diffuse pericellular reticulin staining.