Answer: Mixed epithelial and stromal tumor of kidney

Histology: none provided

Discussion: The tumor exhibits morphologic features of both cystic nehproma and mixed epithelial and stromal tumor (MEST) of the kidney. It contains a dominant (12.5 cm) cyst that has no lining and is filled with hemorrhagic acellular material. The surrounding smaller cysts are lined by a single layer of flat epithelial or hobnail cells. The intervening stroma is fibrotic with focal areas showing hypercellular, ovarian-type stroma. Scattered aggregates of smaller tubular structures with surrounding cellular stroma are also present. Areas with multiple, small to medium sized cysts and scant fibrotic intervening stroma represent features typically seen in cystic nephroma, whereas areas with more abundant stroma with focal small tubular structures are more often seen in MEST. The morphologic features of these two rare benign neoplasms show significant overlap and thus in selected cases the distinction between these two entities
can be difficult.

Reference(s):
– Am J Surg Pathol, vol 31;No4:489-500; Apr. 2007.