Kidney carcinoid tumor: Histological, immunohistochemical and ultrastructural features

Primary carcinoid tumor of the kidney is uncommon. Only limited morphologic studies have been reported in the literature. In general, renal carcinoid tumor reveals typical morphological features as present in carcinoid tumors in the other organs, i.e. growing as trabeculae intermixed with nests of monotonous cuboidal cells that show "salt and pepper" nuclear appearance. As reported in other organs, different morphological expressions may also present in renal carcinoid tumors, such as papillary, pseudoglandular or solid growth patterns and clear/eosinophilic cytoplasmic changes. These morphological variations may mimic other benign and malignant primary neoplasms in the kidney. Therefore, in some cases, ancillary studies are necessary or even crucial to differentiate carcinoid tumors from other primary renal neoplasms. Herewith, we report two primary renal carcinoid tumors with long follow-up, one showed typical morphological features of carcinoid tumor, and the second one revealed some uncommon histological findings. To differentiate these carcinoid tumors from other primary renal neoplasms and confirm neuroendocrine differentiation, immunohistochemical studies are valuable. In case 2 immunohistochemical stain results were not definitive and electron microscopy was performed confirming the diagnosis of carcinoid tumor.