Ultrastructural diagnosis of pleural tumors.

The significant association of two kind of mesothelial cells, differentiated and fibroblastoid, caracterises the ultrastructure of malignant pleural mesotheliomas. The differentiated mesothelial cancer cells (MCC's), whether they are observed in fluids or in biopsies, are not caracteristic enough to assert the diagnosis of mesothelioma. They can easily be mistaken for metastatic cells. Therefore, the diagnosis of mesothelioma relies on the identification of fibroblastoid MCC's, whether they are in association with the preceeding form in epithelial-like tumors, or the main component in sarcomatous-like tumors. Since the absence of fibroblastoid MCC's would eliminate the diagnosis of mesothelioma, patient and careful search for these cells must be done in all pleural tumors with a unique differentiation (epithelial-like or sarcomatouslike). This justifies the need for electron microscopy in the diagnosis of these pleural tumors: for this reason, pleural biopsies should routinely be embedded for possible future ultrastructural examination.