Typing of intermediate filaments in malignant fibrous pleural tumors

We report two cases of a malignant fibrous pleural tumor, one of the localized type and one of the diffuse type. In both cases, typing of intermediate filaments by immunofluorescence microscopy showed that the tumor cells were positive for vimentin and negative for (cyto)keratin and desmin. This result supports the concept that malignant fibrous pleural tumors do not arise from the (cyto)keratin-positive pleural mesothelium but from the submesothelial fibrous tissue. Thus, instead of the usual term "malignant fibrous mesothelioma", the term "malignant submesothelial fibrosarcoma" should be preferred.