Answer: Carcinoid tumor

Histology: The morphology of the tumor is somewhat unusual in that in areas, the tumor cells have prominent, somewhat granular amphophilic cytoplasm. The tumor has a rich vascular network and is organized in sheets and variable sized nests. There is abundant hemorrhage and hemosiderin deposition. The mitotic rate is less than 2 per 10 high power field and there is no necrosis. Based on the histologic appearance the differential diagnosis includes a carcinoid tumor and paraganglioma. Immunohistochemical stains for chromogranin and synaptophysin show diffuse immunoreactivity in the neoplastic cells. Keratin stains (AE1/AE3 and CAM5.2) demonstrate immunoreactivity in a perinuclear dot-like pattern. An S100 immunohistochemical stain demonstrates scattered immunoreactivity. These findings support the above diagnosis.

Discussion: Carcinoid tumors are epithelial neoplasms that are characteristically keratin positive, whereas paragangliomas are typically keratin negative. Both tumors can be positive for neuroendocrine markers. In paragangliomas, an S-100 stain highlights the small sustentacular cells at the periphery of the nests, whereas in carcinoid tumors it is usually negative or focally positive (as in our case). The lack of necrosis and low mitotic activity supports the diagnosis of a typical carcinoid tumor of the lung.