Answer: Metanephric adenoma

Histology: This is a highly cellular lesion composed of small cells with minimal cytoplasm. The cells form small tubules and glomerular structures in a hyalinized stroma. There is abundant psammomatous calcification. The lesion is unencapsulated and directly touches the adjacent non neoplastic kidney. The cells show no mitotic activity, and have cuboidal shape throughout. The lesional cells are diffusely immunoreactivity for WT-1, supporting the diagnosis of metanephric adenoma.

Discussion: A Wilms tumor would show a similar immunophenotype; however, the cells would feature elongated nuclei with mitotic activity. Metanephric adenofibroma would also contain a bland spindle cell stroma which is identical to that of metanephric stromal tumor. The solid variant of papillary renal cell carcinoma would not label for WT-1.