Answer: High grade papillary urothelial carcinoma with invasion

Histology: The lesion consists of atypical, pleomorphic urothelial cells with exophytic growth. Abundant mitoses and areas of necrosis are also seen. The exophytic component is a high grade papillary urothelial carcinoma. Foci of urothelial cells can be seen within the kidney. The invasive foci consist of small irregular nests involving the renal parenchyma.

Discussion: A low power view of the lesion brings to mind a cystic process, raising the possibility of a cystic renal cell carcinoma or an atypical renal cyst. However, closer inspection shows that the 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.

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. Patients with tumors confined to the renal tubules and collecting ducts have better prognosis compared to patients with invasion of the renal parenchyma. Differentiating spread into tubules from true invasion may be difficult. Seeing individual tumor cells or small irregular nests of tumor cells within the renal parenchyma is helpful in establishing a diagnosis of invasion. In contrast, tumor involving tubules without invasion would appear as round nests of distended tubules which follow the contour of the normal tubules and collecting ducts.