Electron microscopy in selection of patients with small cell carcinoma of the lung for medical versus surgical therapy.

Fifty-one cases of small cell carcinoma of the lung were studied by electron microscopy in order to determine if ultrastructural subsets could be found and if these subsets predicted clinical behavior. All of these cases were considered bona fide small cell carcinoma of the lung by light microscopy. Tumors with ultrastructural features of epithelial differentiation were defined by the presence of well-formed, classic desmosomes joining adjacent cells and by additional features of squamous or glandular differentiation. Thirty-one tumors (60%) were considered "typical oat cell" by electron microscopy and 20 (40%) showed features of epithelial differentiation. Fifteen (75%) tumors with epithelial features were considered operable and nine (45%) were resected with curative intent. In contrast, 26 (84%) tumors considered typical oat cell by electron microscopy presented with extensive metastatic disease. The cancer-free 5-year actuarial survival rate of patients whose tumors showed features of epithelial differentiation was 25%. The actuarial survival rate of nine patients who underwent resection of tumors with epithelial features was 38% at 5 years. Only one patient whose tumor was considered typical of oat cell carcinoma by electron microscopy survived 5 years. Our current recommendation is to remove all clinically resectable pulmonary neoplasms with the expectation that these localized small cell tumors are likely to show epithelial features by electron microscopic analysis.