Answer: Solitary fibrous tumor

Histology: The tumor is composed of bland spindled cells embedded in a matrix of dense hyaline collagen. Some areas of the tumor are markedly hypocellular and predominantly collagenous, while other areas demonstrate greater cellularity. The cells are somewhat wavy in the background of dense collagen. A small focus of the tumor demonstrates higher cellularity and nuclear atypia. However, mitotic activity and prominent nucleoli are not identified. The tumor labels intensely for CD34 but not for calretinin. The more cellular area with atypia did not demonstrate increased labeling with Ki-67.

Discussion: Fibroblastic mesothelioma can be very bland cytologically, and difficult to distinguish from simple fibrosis. Mesothelioma should label for cytokeratin and calretinin, though not for CD34. Schwannoma can demonstrate wavy nuclei, and nuclear atypia like that seen in solitary fibrous tumor; however, schwannomas label intensely for S100 protein and not for CD34. Apical scars are located in the upper lobe of the lung and demonstrate prominent elastosis. These are theorized to arise from areas of hypoprofusion.

While classically subpleural in their location, solitary fibrous tumors have now been described in a wide variety of extra-serosal locations. These tumors likely arise from mesenchymal cells which are broadly distributed. Tumors may show a variety of histologic patterns including a short storiform (patternless) pattern, hemangiopericytoma-like pattern, palisading pattern, and diffusely sclerosing pattern. These tumors typically label intensely for CD34 and for Bcl-2, but not for c-kit (CD117). Factors associated with malignant behavior include invasion of the lung parenchyma, chest wall or diaphragm, high mitotic activity, extensive necrosis, and significant cellular atypia. Pedunculated tumors usually behave in a benign fashion, even in the presence of cytologic atypia as is seen in the current case.