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To evaluate the usefulness of an immunohistologic approach to the differential diagnosis of mesothelioma and pulmonary adenocarcinoma, the authors studied paraffin-embedded, fixed tissue sections from 50 primary adenocarcinomas of the lung and 28 mesotheliomas of the pleura by using a panel of monoclonal antikeratin, antihuman milk fat globule (HMFG-2), anti-Leu M1, and monoclonal anticarcinoembryonic antigen (CEA) antibody; we also used a conventional heterologous anti-CEA antiserum with and without prior absorption with spleen powder to remove antibodies to nonspecific cross-reacting antigen (NCA). Keratin was present in both mesotheliomas and adenocarcinomas and did not help in distinguishing between these two neoplasms. HMFG-2 was detected in 48 (96%), and Leu M1 was positive in 47 (94%) of the adenocarcinomas, but not in any of the mesotheliomas. By using conventional rabbit antiserum, the authors detected CEA in the majority of adenocarcinomas (96%), but also in two cases of mesothelioma. When the anti-CEA antiserum was absorbed with NCA, the number of positively reacting adenocarcinomas decreased considerably to 76%; however, after this treatment, none of the mesotheliomas gave positive reactions. The monoclonal anti-CEA antibody was reactive in 36 of the adenocarcinomas (72%), but in none of the mesotheliomas. Our results indicate that, in addition to HMFG-2 and CEA, the expression of Leu M1 antigen by most primary pulmonary adenocarcinoma (94%) and its absence in mesothelioma could be used as a valuable marker for primary adenocarcinoma of the lung that involves the pleura and permits its differentiation from mesothelioma.
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