Answer: Metanephric adenoma

Histology: This needle biopsy of the kidney shows, at low magnification, basophilic tubules set in a myxoid stroma. At high magnification, the tumor is predominantly composed of tubules with focal papillations. The cells have very scant cytoplasm with round monotonous nuclei without visible nucleoli. Mitotic figures are not identified.

Discussion: Collecting duct carcinomas are predominantly composed of tubules within a desmoplastic stroma. Collecting duct carcinomas are one of the more aggressive renal neoplasms and consist of cells with much greater degree of cytologic atypia. The presence of papillary formations within metanephric adenoma may be confused with papillary renal cell carcinoma. However, even Type 1 papillary renal cell carcinomas that are composed of relatively bland nuclei show a greater degree of cytologic atypia than metanephric adenoma. More importantly, even papillary renal cell carcinomas with relatively scant cytoplasm, are not as basophilic, due to more appreciable cytoplasm than that which is seen in metanephric adenoma. Some cases of metanephric adenoma may show transitional areas to cytology seen in Wilms’ tumor. Typically, Wilms tumors have more hyperchromatic pseudostratified nuclei that are larger with more frequent mitotic figures.

Metanephric adenomas are entirely benign lesions. Only a single case has been reported associated with a malignant spindle cell component, designated as a metanephric adenosarcoma. A useful adjunct for the diagnosis of metanephric adenoma is its negativity for EMA staining in contrast to renal cell carcinoma. Establishing the diagnosis of metanephric adenoma on needle biopsy allows either a conservative treatment plan of watchful waiting or potential less aggressive surgery such as a wedge resection or partial nephrectomy.