Answer: Mixed epithelial and stromal tumor/cystic hamartoma of renal pelvis

Histology: The tumor is predominantly cystic, and the cysts are lined by large polygonal eosinophilic cells with nuclei protruding into the cyst lumen. These cells are typical of collecting duct epithelium showing a “hobnail” pattern. The septa are predominantly fibrous, but in areas contain ovarian-like stroma and bland tubules containing eosinophilic secretions. These findings are typical of mixed epithelial and stromal tumor of the kidney.

Discussion: Renal cell carcinomas of the conventional type are commonly cystic, and when the gross appearance is purely cystic and mimics cystic nephroma, the diagnosis of multilocular cystic renal cell carcinoma is appropriate. Such renal cell carcinomas should have at least focal areas of clear cell growth within their septa, which was not seen in this case. Collecting duct carcinoma is an ill-defined entity, but generally is described as featuring high-grade nuclei with tubular growth patterns, stromal desmoplasia, and infiltrative borders. A circumscribed cystic mass as is described in the current case would not be compatible with that diagnosis. Cystic dysplasia is generally seen in the malformed kidneys of neonates. Cystic dysplasia is characterized by primitive ducts in which mesenchyme encircles tubular epithelium, along with bars of cartilage.

The mixed epidermal stromal tumor is also known in the literature as cystic hamartoma of the renal pelvis or genital duct choristoma. It has also been inappropriately named “adult mesopblastic nephroma.” This tumor typically affects middle aged females and presents as a pelvic-centered renal mass. The stroma of these lesions is immunoreactive for estrogen receptor, suggesting a hormonal influence. Highly cystic examples of this lesion like the current case overlap with cystic nephroma of adults, which also predominantly affects females.