Primary renal carcinoid tumor: A rare cystic renal neoplasm

Figure 4 Carcinoid tumors of the kidney are composed of nests and cords of neuroendocrine cells that are noticeable under microscopy. Small- to intermediate-sized tumor cells have uniformly round nuclei with net formation, abundant cytoplasm, and prominent rosette-like structures (A, HE stain, ×200). The neoplasm had low mitotic activity (1/10 HPF). Immunohistochemical studies were performed, and the tumor cells tested strongly positive for the neuroendocrine markers (×200), CD99 (B), chromogranin (C), and vimentin (D). The cells were negative for cytokeratin, the marker for renal cell carcinoma (not shown). Microscopic and immunohistochemical findings were consistent with a well-differentiated neuroendocrine tumor, so called carcinoid tumor.