Answer: Hemangioblastoma

Histology: The lesion is vascular with numerous thin-walled vessels evenly distributed throughout. The vessels appear unremarkable with normal appearing endothelial cells. In between the vessels is a proliferation of short spindle and ovoid cells with pale to clear vacuolated cytoplasm. The lesion lacks significant pleomorphism. Mitotic figures are not identified. The nuclei show some variation in size and shape and minimal nuclear irregularity. Prominent nucleoli are not identified.

Discussion: Hemangiopericytomas share the vascular pattern seen in hemangioblastomas with the exception that some hemangiopericytomas have more dilated and branching vessels (staghorn vessels). In hemangiopericytomas, the cells between the vessels have a more spindled component and do not have the vacuolated clear cytoplasm seen within hemangioblastoma. Hemangiomas lack the intervening foamy cells seen in hemangioblastoma. Metastatic renal cell carcinoma may bear a close resemblance to hemangioblastoma. Furthermore, in patients with von Hippel Lindau syndrome, patients are at increased risk of both hemangioblastoma and renal cell carcinoma. In the current case, the cells between the vessels have a more lipidized appearance than typically seen in renal cell carcinoma. Also, in renal cell carcinomas with more prominent cytologic atypia, prominent nucleoli tend to be visible. In cases where the distinction can not be made with certainty on H&E stain, immunohistochemical stains for epithelial membrane antigen may be helpful where they are negative in hemangioblastoma and positive in renal cell carcinoma.