Answer: Glomus tumor

Histology: The tumor consists of sheets and nests of round to oval cells with a uniform appearance. The neoplastic cells have well-defined cell borders. The nuclei are uniformly round and have no to minimal atypia with a vesicular chromatin pattern and occasional single nucleoli. The cytoplasm is moderate and densely eosinophilic. Immunohistochemically, the tumor cells were immunoreactive for smooth muscle actin and calponin with stains for type IV collagen demonstrating collagen investing individual cells. The tumor cells were negative for desmin, cytokeratin 7 and 20, AE1/AE3, synaptophysin, chromogranin, CD56, S-100 protein, and EMA. In addition, an immunostain with the nuclear proliferation marker Ki-67 revealed approximately 10% positivity. These findings in conjunction are diagnostic of a glomus tumor.

Discussion: The main differential diagnostic consideration for a renal glomus tumor includes other perivascular tumors (hemangiopericytoma, juxtaglomerular tumor), smooth muscle neoplasms (epithelioid leiomyoma), carcinoid tumor, and paraganglioma. Glomus tumors can have a hemangiopericytomatous (staghorn) vascular pattern. However, hemangiopericytomas are negative for actin and positive for CD34 and lack the uniform round cells that are seen in glomus tumors. Juxtaglomerular cell tumor is a rare renal tumor of the young adult, usually presenting with severe hypertension caused by excessive renin secretion. Their histology may resemble that of hemangiopericytoma and glomus tumors. The stroma contains chronic inflammatory cells with a main component of mast cells. The tumor cells are usually immunoreactive for CD34 and actin. At the ultrastructural level, the presence of rhomboid-shaped renin protogranules is diagnostic. However, this tumor occurs in the cortex and not in the pelvis of the kidney. The presence of uniform cells with central nuclei, clear to eosinophilic cytoplasm, and occasional organoid growth pattern also suggests the diagnosis of carcinoid tumor or paraganglioma. The immunohistochemical staining pattern usually assists in differentiating these entities: glomus tumors show positivity for actin, whereas tumors with neuroendocrine features show positivity for neuron-specific enolase and chromogranin. In addition, glomus tumors show prominent intercellular basal lamina outlining individual or small group of cells. Glomus tumors are usually negative for all neuroendocrine markers. This case is the first to document a glomus tumor within the kidney in peer-reviewed literature. In the present case, the benign cytohistologic features, including uniform appearance with minimal atypia and low mitotic rate, as well as a complete surgical removal would be expected to be associated with an excellent clinical outcome.