Answer: Malignant mesothelioma

Histology: The lesion is a fibrous tumor primarily in the pleura. The tumor infiltrates into surrounding adipose tissue. The tumor is composed primarily of an epithelial proliferation that grows in sheets and cords of cells. In some areas the tumor cells appear to grow in a glandular pattern. The neoplastic cells have prominent nucleoli and eosinophilic cytoplasm. Mitotic figures are easily identified.

Discussion: The major differential diagnosis in this case is between a metastatic adenocarcinoma vs. a malignant mesothelioma. The clinical history is, of course, helpful in this differential diagnosis. In the absence of a lung mass or known primary, the presence of a lesion primarily in the pleura would favor a diagnosis of mesothelioma. Immunohistochemical studies can also be extremely helpful in making this distinction. While both mesothelioma and metastatic adenocarcinomas are positive for cytokeratin, a mesothelioma would also stain for calretinin and would be negative for CD15 and CEA. Tumor cells of the solitary fibrous tumor of the pleura stain strongly for CD34 and therefore negative stain for CD34 in this case would rule out the diagnosis of solitary fibrous tumor.