Answer: Papillary renal epithelial neoplasm of low malignant potential

Histology: The tumor is composed of small, low cuboidal, primitive appearing cells with minimal cytoplasm. Focally, the cells form tubules, and foam cells are present within the lesion. Rare mitoses are evident but the mitotic rate is not high. The lesion has an encapsulated border. On immunohistochemical analysis, the tumor cells are diffusely immunoreactive for both cytokeratin 7 and EMA.

Discussion: An epithelial predominant Wilms tumor would be unusual in an adult of this age. It would feature tubules composed of more columnar cells with hyperchromatic nuclei, high mitotic rate, and necrosis. A metanephric adenoma is the main differential diagnosis in this case. A metanephric adenoma could look morphologically identical to this lesion, with the exception that it is unencapsulated and typically lacks foam cells. The distinction is often best made on immunohistochemical analysis; metanephric adenomas label for WT-1 protein, in contrast to papillary renal epithelial neoplasms. Also, metanephric adenoma is only focally positive for cytokeratin 7, and completely negative for EMA. Ewing sarcoma/primitive neuroectodermal tumor is one of the adult renal lesions which commonly mimics Wilms tumor. The diagnosis is excluded in the current case by the finding of true tubular differentiation, and a paucity of mitotic figures and necrosis.

While classically papillary renal cell carcinoma requires over 75% papillary formations, the solid variant of papillary renal cell carcinoma has the same immunophenotypic and genetic (trisomies of chromosome 7 and 17) abnormalities. Importantly, the solid variant of papillary renal cell carcinoma creates a great differential diagnosis for metanephric adenoma. In the current case, the lesion is of low nuclear grade, and measures less than three centimeters, and hence can be placed under the general category of renal epithelial neoplasm of low malignant potential.