Answer: Metanephric adenoma

Histology: At low magnification this lesion has an extremely basophilic appearance. Although the lesion is circumscribed, it lacks a fibrous capsule. The lesion consists of small tubular structures lined by, in areas, bland cuboidal nuclei. The stroma has an edematous appearance. In other areas there are tubules, which are lined by taller stratified columnar epithelium with more appreciable mitotic activity. Scattered calcifications are noted.

Discussion: Although some of the tubules lined by more columnar epithelium with mitoses, out of context could be consistent with a Wilms’ tumor. The overall morphology of this lesion is inconsistent with a Wilms’ tumor. Wilms’ tumors have a fibrous capsule surrounding them which is lacking in the current case. Furthermore, the atypia seen within only scattered tubules in the current case would be more widespread throughout the lesion in a Wilms’ tumor. The presence of small rounded tubules lined by cuboidal cells without mitoses are classic for metanephric adenoma and not seen within a Wilms’ tumor. Both cortical adenomas and renal cell carcinomas can be differentiated from metanephric adenomas even at low magnification. Only metanephric adenoma has this extremely basophilic appearance due to the inconspicuous cytoplasm. Classic metanephric adenomas are entirely benign. We would expect the current case to also have a benign clinical course although the presence of mitoses and some cytologic atypia would warrant follow-up of the patient.