Answer: Metanephric adenoma

Histology: The lesion is overall circumscribed with a sharp interface between the kidney and lesion although no capsule is present. Areas of the tumor consist of small tubules lined by bland nuclei with nuclear grooves. There is very scant cytoplasm giving the lesion an overall very basophilic appearance. Scattered calcifications are noted. Elsewhere the tumor has a prominent papillary growth pattern. The nuclei lining the papillae are similar to that seen in the tubules without cytologic atypia and without any mitotic figure and very scant cytoplasm.

Discussion: One of the more common differential diagnoses in renal pathology is between metanephric adenoma and papillary renal cell carcinoma. The distinction is critical in that metanephric adenoma is one of the few benign epithelial neoplasms of the kidney. This lesion was 1.8 cm in diameter which if it were a papillary neoplasm would exclude a papillary adenoma since by definition papillary adenomas must be less than 5 mm. In terms of the distinction between papillary renal cell carcinoma and metanephric adenoma, it can usually be made at scanning magnification. Metanephric adenoma, as seen in this case, has a very basophilic appearance due to the extremely scant cytoplasm whereas papillary renal cell carcinoma even those with more basophilic cytoplasm, appear different at low magnification. At higher magnification, metanephric adenomas have no cytologic atypia and virtually no mitotic figures whereas papillary renal cell carcinomas, in addition to having more cytoplasm, typically show greater nuclear atypia. The more usual pattern of metanephric adenoma is tubules set in a fibromyxoid background as seen in much of this lesion. The unusual feature in this case is the striking papillary component. While small papillae may be seen in the typical metanephric adenoma it is unusual to have a prominent papillary pattern as seen in the current case. However, given the similar cytology between the areas of papillary formation and tubule formation, the entire lesion is consistent with a metanephric adenoma. Scattered calcifications are also typical of metanephric adenoma. In difficult cases one can use immunohistochemical stains to help resolve the differential diagnosis. Given the peculiar nature of this lesion, stains were performed at our institution showing both areas of the tumor cells to be positive for WT-1 and negative for CK7 which confirms the above diagnosis. A reverse staining with positive CK7 and negative WT-1 would be seen in papillary renal cell carcinomas.