Cytopathology of Oncocytic Carcinoid Tumor of the Lung Mimicking Granular Cell Tumor

Background The cytopathologic features of oncocytic carcinoid tumor of the lung, a rare variant of carcinoid tumor that is composed exclusively of oncocytes, have not been described before in detail. Case The bronchial brush smears from an 80-year-old female with an endobronchial obstructive tumor showed single and loose clusters of tumor cells with abundant granular, eosinophilic cytoplasm. The differential diagnoses included oncocytic carcinoid tumor, granular cell tumor, other oncocytic tumors of bronchial origin and metastatic oncocytic tumors. Immunocytochemistry and electron microscopy confirmed the diagnosis of oncocytic carcinoid tumor. Conclusion Oncocytic carcinoid tumor of the lung has cytopathologic features similar to those of granular cell tumor and pulmonary oncocytoma. Immunocytochemistry, electron microscope or both are necessary to distinguish these neoplasms.