Answer: Urothelial carcinoma and oncocytoma

Histology: There are 2 separate and distinct populations of tumor cells in this case. One tumor consists of eosinophilic cells with small round and regular nuclei arranged in nests, cords and tubules. The background shows focal myxoid changes. 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.

On low magnification, the second lesion is centered in the renal pelvis. It consists of atypical, pleomorphic urothelial cells with exophytic growth. Abundant mitoses and areas of necrosis are also seen. The degree of pleomorphism, mitoses and necrosis is consistent with a high grade papillary urothelial carcinoma.

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.

The second tumor is located in the renal pelvis and is lined by neoplastic urothelial cells, and not the clears cells that would be seen in a renal cell carcinoma or hobnail cells that are seen in an atypical renal cyst. It does not show the cytologic features of the oncocytoma and the location in the renal pelvis is also unlikely for an oncocytoma. Papillary urothelial carcinomas in the renal pelvis may involve the kidney in two ways. Carcinoma cells may spread into the collecting ducts and tubules from the renal pelvis, and not directly invade the renal parenchyma. The other way it involves the kidney is by direct invasion into the renal parenchyma. This urothelial carcinoma was non-invasive.