REACTIVE MESOTHELIAL HYPERPLASIA VS MESOTHELIOMA, INCLUDING MESOTHELIOMA IN-SITU - A BRIEF REVIEW

In biopsy tissue, discrimination between reactive mesothelial hyperpla sia and epithelial mesothelioma can pose a major problem for the surgi cal pathologist. Confidence in the diagnosis is often proportional to the amount of tissue available for study and depends largely on findin gs of invasion and the extent and cytologic atypia of the lesion, beca use there is no marker specific for the mesothelium and that discrimin ates consistently among normal, hyperplastic, and neoplastic mesotheli al tissue. Therefore, mesothelioma in situ is diagnosable only when in vasive epithelial mesothelioma is demonstrable in the same specimen, i n a follow-up biopsy specimen, or at autopsy. Comparison of 22 cases o f mesothelioma in situ that fulfill these requirements for diagnosis w ith 141 invasive mesotheliomas and 78 reactive. mesothelioses indicate s that strong linear membrane-related labeling for epithelial membrane antigen and silver-labeled nucleolar organizer region-positive materi al that occupies 0.6677 mu m(2) or more of the nucleus in an atypical in situ mesothelial lesion of the pleura are found consistently in neo plastic mesothelial cells. Although these findings may engender suspic ion of mesothelioma in situ in high-risk persons, the criteria for dia gnosis of pure mesothelial lesions of this type are still under study. Mesothelioma in situ should be considered proved only when unequivoca l invasion is identified in a different area of the pleura or at a dif ferent time; a diagnosis of pure mesothelioma in situ should not be ma de in patients not exposed to asbestos.