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 are found in some lesions.

Discussion: Metanephric adenomas are benign epithelial renal tumors that are predominantly encountered in the fifth and sixth decades of life. Many are found incidentally. Others, present with hematuria, flank pain and or polycythemia. The differential diagnosis of this lesion include solid variant of papillary renal cell carcinoma (SPRCC). Distinction from SPRCC can be more difficult when faced with a limited needle biopsy sample. Subtle morphologic features such as the presence of foamy macrophages and a more open nuclear chromatin pattern in SPRCC can be of some utility. In difficult cases, immunohistochemistry can help distinguish the two entities. Metanephric adenoma is WT1 positive and EMA/CK7 negative in contrast to the WT1 negative and EMA/CK7 positive staining profile of SPRCC.

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