The unusual spectrum of neuroendocrine lung neoplasms.

Neoplasms of the lungs showing neuroendocrine differentiation are classified histologically into the following groups: (1) carcinoid, (2) atypical carcinoid (well-differentiated neuroendocrine carcinoma and malignant carcinoid, (3) small cell neuroendocrine carcinoma (small cell undifferentiated carcinoma and oat cell carcinoma), and (4) large cell neuroendocrine carcinoma (atypical endocrine tumor of the lung and intermediate neuroendocrine carcinoma). Nine examples of neuroendocrine lung carcinomas are discussed that have unusual histologic features that make it difficult to assign them to one of the above groups, have unusual immunohistochemical features, have unusual ultrastructural features, or exhibit a biologic behavior different from what one would have predicted from their morphologic appearance. The findings in these nine cases suggest that the present classification of neuroendocrine lung neoplasms may be too precise and that these neoplasms, like other nonneuroendocrine pulmonary tumors, exhibit a wider morphologic and biologic spectrum than previously appreciated.