Answer: Sarcomatoid urothelial carcinoma

Histology: This lesion shows several different patterns. The tumor consists of small and larger irregular nests, some with central necrosis, with an infiltrative pattern extending between normal renal tubules and glomeruli. The tumor is pleomorphic with numerous mitotic figures, and in areas has clear cytoplasm. Within the medulla, the collecting ducts are filled with similar malignant cells. Elsewhere, the tumor assumes a more spindled morphology associated with extensive necrosis and a desmoplastic reaction.

Discussion: This case was sent in for consultation with the presumptive diagnosis of a sarcomatoid clear cell renal cell carcinoma. When considering a poorly differentiated carcinoma within the kidney, one must also rule out the possibility of a urothelial carcinoma arising from the renal pelvis extending into the kidney. The presence in the current case of nests of cells with clear cytoplasm creates further diagnostic confusion, as it is typically associated with renal cell carcinoma. However, urothelial carcinoma may uncommonly have clear cytoplasm. The presence of a nested pattern of tumor is much more typical of a urothelial carcinoma as contrasted to clear cell renal cell carcinoma or any other subtypes of renal cell carcinoma. A very useful feature in this case is the extension of urothelial carcinoma of the collecting ducts. This would not be seen with clear cell renal cell carcinoma. Whereas collecting duct carcinoma of the kidney may show an in situ component within the collecting ducts, it typically would not consist of solid nests of tumor replacing the collecting ducts as seen in the current case. Furthermore, the lesion lacks any of the tubular formation typical of collecting duct carcinoma. One must be aware, however, that collecting duct carcinomas may show, in addition to tubular differentiation, urothelial differentiation.