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Solitary Fibrous Tumor of Pleura: Expression of Cytokeratins

&NA; A case of solitary fibrous tumor of the pleura (SFT) is presented. The histogenesis of this uncommon tumor is debated with most investigators favouring origin in submesothelial fibroblasts. Part of the evidence supporting this has been the persistent negativity of the tumor cells for cytokeratin — a feature militating against origin in mesothelial‐lining cells. Our case shows unequivocal focal cytokeratin positivity in tumor cells; we feel that although this indicates mesothelial differentiation it does not militate against origin in submesothelial fibroblasts since, in reactive conditions, these are capable of mesothelial differentiation including expression of cytokeratin. Indeed, it reinforces the hypothesis that SFT is of submesothelial origin. Solitary fibrous tumors can be cellular and atypical. The reactivity of the tumor with cytokeratin, albeit rarely, should be considered in differentiating SFT from sarcomatoid mesothelioma.