Basal lamina reduplication in malignant epithelioid pleural mesothelioma

Malignant mesothelioma of the pleura is divided in three morphological vari ants: epithelioid, sarcomatous, and biphasic. Histological similarities bet ween epithelioid malignant mesothelioma (EMM) and lung adenocarcinoma are r esponsible for the difficult differential diagnosis. Monoclonal antibodies are useful for distinguishing the two neoplasms through immunohistochemical phenotyping, although many cases require ultrastructural characterization for definitive diagnosis. In this study, transmission electron microscopic observations of EMM were compared with those of peripheral adenocarcinoma o f the lung (PAL). More specifically, the morphology of the basal lamina is described in 23 cases of EMM and 12 cases of PAL. Reduplication of the basa l lamina (RBL) was found in 11 cases (48%) of EMM and in none of the PAL ca ses. The same cases were immunostained for type IV collagen and the localiz ation of this basement membrane component corresponded to the areas where b asal lamina was observed. Since RBL has been associated with neoplastic dif ferentiation in other tumors, this novel feature in EMM needs to be evaluat ed in future prognostic studies in malignant mesothelioma of the pleura. Mo reover, RBL expression in EMM may be an additional ultrastructural paramete r used in the differential diagnosis between EMM and adenocarcinoma.