Tumors of serosal tissue origin.

Primary serosal neoplasms demonstrate a wide spectrum of growth patterns and biologic aggressiveness. The adenomatoid tumor is uniformly benign, whereas the diffuse malignant mesothelioma pursues a downhill clinical course, rapidly leading to fatality. The cystic peritoneal mesothelioma occupies an intermediate position characterized by persistent and/or recurrent disease but without progression to death. The distinction of an epithelial mesothelioma from metastatic adenocarcinoma remains a challenging problem. In the vast majority of cases, this can be accomplished by combining routine histochemistry, immunocytochemistry, and electron microscopy. The absence of epithelial mucins and nonreactivity with antibodies to CEA strongly favor mesothelioma. Ultrastructurally observed long, thin, sinuous surface microvilli without a glycocalix, well-developed desmosomes, and abundant tonofilaments add further support for a primary serosal neoplasm. The sarcomatoid mesothelioma can easily be confused with a chest-wall sarcoma. Despite lacking ultrastructural evidence of "epithelial" differentiation, immunocytochemical studies demonstrate cytokeratin. This distinguishes the sarcomatoid mesothelioma from most soft-tissue sarcomas. There remains a small number of cases, particularly those in the "poorly differentiated" or "transitional" category, in which the distinction between mesothelioma and metastatic carcinoma remains difficult. In this situation, it is imperative that all the clinical information be closely reviewed and a diligent search for a primary site be carried out. There are many parallels between reactive and neoplastic serosal tissue. The desmoplastic/sarcomatoid mesothelioma morphologically and immunocytochemically resembles the reactive multipotential subserosal cell (MSC) of injured serosal tissue, whereas the adenomatoid tumor, cystic peritoneal mesothelioma, and epithelial mesothelioma resemble surface mesothelium. The poorly differentiated mesothelioma resembles a stage of maturation between the two extremes, and thus the term "transitional" mesothelioma is suggested. The localized fibrous tumor of the pleura is unique among all other serosal neoplasms in its failure to express cytokeratin. It more closely resembles the unspecialized connective tissue fibroblast of normal serosal tissue, and thus may be more analogous to a soft-tissue tumor than to the remaining mesothelial-derived neoplasms.