Answer: Carcinoid tumor of the lung

Histology: Sections show a bronchocentric mass that is almost entirely obstructing the bronchial lumen. In the bronchial lumen admixed with the nests of tumor cells and in the surrounding lung parenchyma there are sheets of inflammatory cells including numerous lipid laden macrophages. The tumor cells have small, round to oval to spindle shaped nuclei with fine dispersed chromatin and no or small inconspicuous nucleoli, and scant eosinophilic cytoplasm. The associated mixed inflammatory infiltrate is a secondary response to the intraluminal obstruction caused by the tumor.

Discussion: The differential diagnosis of carcinoid tumors include other neuroendocrine tumors (atypical carcinoid, small cell carcinoma and large cell neuroendocrine carcinoma), low grade adenocarcinoma, and paragangliomas. Spindle cell carcinoids may be confused with mesenchymal tumors, particularly smooth muscle tumors. Tumors with a prominent papillary pattern may be mistaken for sclerosing hemangioma of the lung. Immuohistochemistry is helpful in separating for carcinoid tumors from tumors with neuroendocrine differentiation.