Answer: Oncocytoma

Histology: The tumor consists of new eosinophilic cells with small round and regular nuclei arranged in nests, cords and tubules. The background shows myxoid changes and focal fibrous scar. On high magnification, the nuclei are all uniformly round and show bland cytology without significant atypia. Small nucleoli may be seen in some of the cells.

Discussion: Oncocytomas are benign renal neoplasms that are derived from intercalated cells. Grossly, the tumor appears as a mahogany, brown mass with a central scar. Histologically, the tumor is composed of eosinophilic cells with bland cytology, having small, round, regular nuclei. The tumor cells are often arranged in nests, cords or tubules in a myxoid background. Occasional degenerative atypia or pleomorphism may be seen, however, oncocytomas will not show any areas of papillary formation, necrosis or clear cells. Oncocytomas may occasionally infiltrate perirenal soft tissue, however, this does not affect the prognosis. Electron microscopy will show that the cells are packed with mitochondria, which accounts for their eosinophilic appearance.

Oncocytomas may resemble chromophobe renal cell carcinomas, in that they have prominent eosinophilic cytoplasm, however, oncocytomas usually show densely eosinophilic cytoplasm throughout the entire lesion and lack the pale appearance seen in many of the cells in a chromophobe renal cell carcinoma. Chromophobe renal cell carcinomas also show perinuclear halos, and nuclear irregularity and pleomorphism, which would not be seen in oncocytomas. Chromophobe renal cell carcinomas also have distinct cell borders, which often resemble plant cells, which is a feature not seen in oncocytomas. A Hales colloidal iron stain, which stains for acidic mucin, can be helpful in cases where it is difficult to tell the two lesions apart. Chromophobe renal cell carcinomas would show intense and diffuse staining within the cytoplasm of the tumor cells, while an oncocytoma is usually negative or only shows luminal staining within some of the tubules, for Hales colloidal iron. Oncocytomas are differentiated from renal cell carcinomas with oncocytic features by the lack of cytologic atypia. Also, in renal cell carcinomas with oncocytic features, there are often areas of more conventional renal cell carcinomas showing clear cells.

Reference(s):
Am J Surg Pathol 1997; 21:1-12.

Am J Surg Pathol 2000; 24:1247-56.