Answer: Tubulopapillary hyperplasia and atypical multilocular renal cyst

Histology: There are 2 lesions seen in this case. One consists of a proliferation of epithelial cells with tubular and papillary architecture. The cells lack cytologic atypia.
The entire lesion measures less than 5 mm. The other lesion consists of a multilocular cyst lined by flatten cuboidal cells and focal cells showing clear cell change. Clear cells are not identified within the wall of the cyst. The background kidney shows marked atrophy of tubules with “thyroidization” and glomerulosclerosis, consistent with an end-stage kidney.

Discussion: The background kidney is consistent with an end-stage kidney and it is likely that the patient has been on renal dialysis. Over 50% of long term renal dialysis patients will have changes of acquired cystic disease of the kidney. These patients often show multiple cysts as well as foci of tubulopapillary hyperplasia or adenoma in the kidneys. The cysts in these patients may be simple, atypical (cytologic and/or architectural) or contain overt renal cell carcinoma. The cyst in this case is best classified as an atypical multilocular cyst, as it shows lining cells with cytologic atypia and clear cell change. When renal cell carcinoma occurs in these cysts, clusters of tumor cells tend to be seen within the wall of the cyst.

Tubulopapillary hyperplasia or adenoma is defined as small proliferation of epithelial cells lacking cytologic atypia with either tubular or papillary architecture that are less than 5 mm. Small amounts of cellular stroma are usually identified within these proliferations. The differential diagnosis of epithelial proliferations with tubular and papillary architecture in the kidney includes renal cell carcinoma, tubulopapillary hyperplasia (also called adenoma) and metanephric adenoma. Metanephric adenoma consists of uniformly small epithelial cells forming very small acini. The tubules and papillae are tightly packed in an acellular stroma. A diagnosis of renal cell carcinoma would be reasonable if the lesion showed more cytologic atypia and a larger size. Immunohistochemical stains may also be helpful, as EMA is seen in renal cell carcinoma and tubulopapillary hyperplasia, while it is negative in metanephric adenoma. In addition, metanephric adenoma, which is histogenetically related Wilm’s tumor, is positive for WT1 and CD 57.