Answer: Solitary fibrous tumor

Histology: The lesion consists of localized mass arising from the pleura. 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.

Discussion: Solitary fibrous tumors may occur in any location, however the classic presentation is that of a circumscribed pleural mass. The origin of this tumor is still debated. Some believe that it arises from fibrous tissue, while others think that it is of mesothelial origin. Solitary fibrous tumors 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. These tumors typically do not show signifcant atypia or abundant mitoses. Solitary fibrous tumors with increased cellularity, pleomorphism and more than 4 mitoses/10 HPF are regarded as malignant variants. Immunohistochemical stains are helpful in differentiating the tumor from other histologically similar neoplasms. Solitary fibrous tumors are negative for calretinin (a mesothelial marker) and cytokeratins, but are positive for CD34 in a diffuse and strongly positive pattern. Although solitary fibrous tumors may show ranching vascular channels like those seen in hemangiopericytomas, hemangiopericytomas often show weak and patchy staining with CD 34.