Papillary carcinoid tumor of the lung.

Carcinoid tumors of the lung have a wide histologic spectrum. The histologic differential diagnosis of papillary tumors in the lung generally does not include carcinoid. A carcinoid tumor that formed a discrete coin lesion on chest radiograph is presented in this report. On gross examination the center appeared and felt spongy. On microscopic examination delicate, compacted papillae were separated by a serpentine space continuous with air spaces at the periphery of the tumor. The papillae were each composed of a fibrovascular core, an undulating basement membrane separating stroma from epithelial cells, and cuboidal clear and dark cells that proved to be, respectively, healthy and degenerate cells of carcinoid tumor with neurosecretory granules. Upon the carcinoid cells, draped in the manner of an umbrella, were nonciliated, respiratory cells that proved to be Clara cells. The various histologic patterns of bronchopulmonary carcinoids and the differential diagnosis of papillary tumors within the lung are tabulated.