Answer: Mixed epithelial stromal tumor (MEST)

Histology: The tumor is fairly well circumscribed, and is both solid and cystic. It does undermine the renal pelvic urothelium. The tumor is biphasic. It consists of a bland spindle cell stromal component of variable cellularity, consisting of fibroblast-like cells, cells resembling ovarian stroma, and smooth muscle cells. There is also a benign-appearing epithelial component, which includes ciliated Mullerian-type epithelium, clusters of small tubular glands with eosinophilic secretions, and elongated tubules reminiscent of fibroadenoma of the breast. These are typical features of the mixed epithelial stromal tumor, also known as cystic hamartoma of the renal pelvis, or genital duct choristoma.

Discussion: Congenital mesoblastic nephroma essentially does not occur over the age of two and a half years. There have been tumors in the literature described incorrectly as “adult congenital mesoblastic nephromas” which appear to be a mixture of different entities. Many of these tumors are in fact MEST. Metanephric stromal tumor is typically seen in young children, though an adult case has been reported. An MST should show more prominent concentric peritubular growth (“onion skinning”), angiodysplasia, juxtaglomerular cell hyperplasia with an entrapped glomeruli, or heterologous differentiation. An MST is a pure stromal lesion that lacks an epithelial component. A cystic nephroma should not have the prominent solid growth of the current lesion, and its epithelial component should be the “hobnail type” epithelium of collecting ducts.

Mixed epithelial stromal tumor typically occurs in middle-aged females. The tumor’s stromal cells typically label for estrogen and progesterone. The tumor is completely benign. Whether it is a hamartoma or a true benign neoplasm remains to be determined.