Metastatic adenocarcinoma of unknown primary site: diagnostic electron microscopy to determine the site of tumor origin.

Poorly differentiated malignant tumors and metastatic adenocarcinomas have been studied by electron microscopy. Rapid ultrastructural analysis allows the accurate diagnosis of the cell of tumor origin to be reported in conjunction with the routine surgical signout of paraffin sections. The identification of specialized intracellular structures is useful for determining the site of origin of metastatic tumors. Particularly useful in this material was the presence of lamellar (surfactant) bodies, typical of alveolar cell carcinoma of the lung, as well as the presence of apical terminal webs, diagnostic of gut epithelial cells.