Answer: Metanephric adenoma

Histology: The well circumscribed unencapsulated tumor is composed of closely packed small tubular structures with little intervening stroma. Tubules are lined by uniform cuboidal cells containing only minimal eosinophilic cytoplasm. The latter together with the high nuclear to cytoplasmic ratio in epithelial lining cells may impart a “primitive/blue cell” falsely alarming morphology. Some of the tubules contain abortive papillae leading to glomeruloid morphology. Mitotic figures, luminal secretions and necrosis are lacking. Psammoma bodies and foamy histiocytes can be found in some lesions.

Discussion: Metanephric adenomas (MA) are benign epithelial renal tumors that are thought to be histogenetically related to Wilms’ tumor (WT). MA has been demonstrated to be morphologically and immunophenotypically identical to maturing WT and nephrogenic rests.

Although MAs are predominantly encountered in the fifth and sixth decades of life, occasional pediatric and young adult examples are on record. Many MAs are found incidentally. Others, present with hematuria, flank pain and or polycythemia.

The differential diagnosis of this lesion includes solid variant of papillary renal cell carcinoma (PapRCC). Distinction from PapRCC can be more difficult when faced with a limited needle biopsy sample or subtle morphologic features. In difficult cases, immunohistochemistry can help distinguish the two entities. Metanephric adenoma is WT1 positive and EMA/CK7/AMACR negative (CK7 can be focally positive) in contrast to the WT1 negative and EMA/CK7/AMACR positive staining profile of PapRCC.

Reference(s):
– Clin Lab Med. 2005 Jun;25(2):379-92. Metanephric neoplasms: the hyperdifferentiated, benign end of the Wilms tumor spectrum? Argani P.
– Am J Surg Pathol. 2001 Oct;25(10):1290-6. Metanephric adenoma, nephrogenic rests, and Wilms’ tumor: a histologic and immunophenotypic comparison. Muir TE, Cheville JC, Lager DJ.