The distinction of mesothelioma from adenocarcinoma in malignant effusions by electron microscopy.

To determine the usefulness of the electron microscopic (EM) differential diagnosis between malignant mesothelioma and metastatic adenocarcinoma in cytologic specimens of serous fluids, we undertook a prospective study of 17 pleural and peritoneal effusions from 14 patients. In the nine effusions identified as malignant by routine cytologic examination, EM correctly diagnosed three mesotheliomas and six adenocarcinomas. EM resolved the differential diagnosis of mesothelioma versus adenocarcinoma in three cases in which routine cytologic examination could not. As with tissue specimens, EM cannot be used to diagnose the malignancy of cytologic specimens; it can, however, reliably identify the origin of cells diagnosed as malignant by routine cytologic examination. We conclude that, when EM is used to evaluate cytologically malignant effusions, it can accurately distinguish mesothelioma from adenocarcinoma. This technique will be diagnostically useful in selected cases and may be helpful in avoiding more invasive procedures as well as delays in diagnosis and therapy.