Answer: Solitary fibrous tumor

Histology: none provided

Discussion: The solitary fibrous tumor in the lung was originally described as solitary or benign fibrous mesothelioma. That term is confusing and is misleading and incorrect, since it now appears that cells of the tumor are basically uncommitted mesenchymal cells in the submesothelial layer of this or the parietal pleura. Further, similar lesions have been described in many other organs of the body and in soft tissue. The characteristic feature is that of alternating areas of fibroblast-like cells squeezed between abundant collagen fibers. There may be a hemangiopericytoma-like pattern, and other solitary fibrous tumors may have myxoid features. Both benign and malignant variants of solitary fibrous tumor have been described; the malignant lesion is much less common and should be diagnosed only when there are some features of more orthodox solitary fibrous tumor in other areas of the neoplasm. The immunocytochemical reaction of solitary fibrous tumor is helpful. The tumor cells are strongly positive for CD34 and for BCL2. This staining pattern is very helpful in the differential diagnosis between Schwannoma (a common error), smooth muscle tumors or sarcomatoid or desmoplastic mesothelioma. Synovial sarcoma may present a problem in more cellular lesions, although cytogenetic features and the presence of cytokeratin-positive areas is helpful in establishing the diagnosis of synovial sarcoma.