Answer: Metanephric adenoma

Histology: The lesion appears to be well circumscribed associated with hemorrhage and sclerosis. The neoplasm itself consists of a basophilic proliferation of tubules. At higher magnification, the cells form atrophic tubules lined by cells with high nuclear-to-cytoplasmic ratio and very scant cytoplasm. The nuclei are uniform, round, lack prominent nucleoli, and have no visible mitotic activity. Scattered luminal calcifications are noted. Some areas of the lesion show a more myxoid background between the tubules. Surrounding the lesion is a dense fibrous capsule with a sharp demarcation from the surrounding normal kidney.

Discussion: Metanephric adenomas more typically occur in women within a wide age range. Similarly, these lesions may vary in size from minute to 15cm. Although these lesions are typically described as being unencapsulated, we have seen numerous examples with a very dense fibrous capsule, often which is calcified. In addition to the tubular formation seen in the current case, one may have glomeruloid and papillary formations, as well. The finding of small calcifications within the lumina is a common finding, as seen in the current case. The most common differential diagnosis is between metanephric adenoma and solid variant of papillary renal cell carcinoma. The differential diagnosis typically occurs in the setting of metanephric adenoma with abortive papillary structures. In large part, the diagnosis can be made at low magnification. Whereas metanephric adenomas appear very basophilic similar to Wilms tumor, papillary renal cell carcinomas have a much paler appearance. This distinction can be made at scanning magnification. Immunohistochemistry can also help in this differential diagnosis with metanephric adenomas positive for WT1 and negative for CK7 and the reverse pattern of staining for papillary renal cell carcinoma. Metanephric adenomas are entirely benign when present in a pure form, although rare examples have been associated with a sarcomatous component. One may also see in adults metanephric adenomas with small foci that resemble Wilms tumor. However, this finding does not rule out the diagnosis of metanephric adenoma and is still associated with an entirely benign prognosis.