Carcinomas Several types of tumor that show neuroendocrine differentiation occur in the lung. They are defined by their growth patterns, cytologic features, immunohistochemical reaction for chromogranin A, the presence of neurosecretory granules by electron microscopy, and the presence of a number of neuropeptides that can be detected biochemically or by immunohistochemical methods. Classification systems attempt to predict behavior on the basis of morphology. A current scheme for classifying tumors is shown in the Table. Carcinoid tumors are low-grade malignant tumors of epithelial cells showing differentiation toward Kulchitsky cells.
Peripheral tumors are usually asymptomatic.
In those presenting as solitary pulmonary nodules, the borders are usually smooth and well-defined. Thus, despite the fact that they form discrete masses, they cannot be removed endobronchially. Peripheral tumors have usually obliterated the airway in which they arose. Arrangement of nuclei at the edge of the nest perpendicular to the margin, i.e., palisading of nuclei, is common. Cells are uniform with a normal nuclear to cytoplasmic ratio, and have round nuclei, stippled nuclear chromatin, and small nucleoli.