The biological implications of bronchial tumors.

Seventy-six consecutively resected primary pulmonary tumors were assessed first by routine light microscopy and subsequently by electron microscopy and immunohistochemical staining techniques to precisely identify features of differentiation. In 66% of the cases, this assessment provided information that modified or revised the histologic diagnosis provided by light microscopy alone. The following conclusions were reached: (1) The term "large cell undifferentiated carcinoma" has been applied to a heterogenous group of tumors, most of which have ultrastructural and immunohistochemical features of differentiation not identifiable by routine light microscopy. (2) Forty percent of the tumors previously called "large cell undifferentiated carcinoma" have predominantly neuroendocrine differentiation and appear to have a clinical course comparable to that of small cell neuroendocrine carcinoma. (3) The majority of pulmonary carcinomas (especially those previously called "poorly differentiated" or "undifferentiated") may simultaneously demonstrate more than one pattern of differentiation when studied by electron microscopy and immunohistochemistry. (4) The frequency of neuroendocrine neoplasms of the lung, as determined by these and previous studies, is considerably greater than suspected on the basis of light microscopic studies alone. These comprise a clinical and morphologic spectrum ranging from bronchial carcinoid to small cell carcinoma, all of which have immunohistochemically demonstrable hormone production, although paraneoplastic hormonal syndromes are manifested in only a small minority of cases.