Neuroendocrine neoplasms of the lung.

Neuroendocrine tumors of the lung continue to be difficult nosologic and diagnostic problems, centering on the time-honored terms of "carcinoid," "atypical carcinoid," and "small cell carcinoma." Problems that are encountered in the classification of such neoplasms revolve around the differing criteria that have been advanced for their definition and variable application of such criteria in common practice. This review considers the epithelial and nonepithelial lesions of the lung that may demonstrate neuroendocrine and neuroectodermal differentiation. A proposal is made for a simplified system of classifying the epithelial tumors, dividing them into 3 grades with appended descriptive modifiers.