Answer: Mixed epithelial stromal tumor

Histology: The lesion is cystic, and the walls are composed of dense fibrous tissue and cellular areas simulating ovarian stroma. Tubules within this stroma have the appearance of thyroid follicles, in that they have dense pink secretions within. The lesion has both solid and cystic areas, and lacks clear cells within its lining. It is well delineated from the surrounding kidney. These are the typical features of a mixed epithelial stromal tumor (MEST).

Discussion: Most cystic renal cell carcinomas typically have nests of clear cells within the septa that the divide the cysts. Segmental polycystic kidney disease consists of cystic dilatations of renal tubules, such that cystic area is ill defined and intermingles with the native kidney. Glomerulocystic disease is also often present. Many authors consider cystic nephroma to be related to mixed epithelial stromal tumor. Other authors consider cystic nephroma to be a highly cystic lesion that lacks the solid areas typically seen in MEST. It has been proposed that these lesions are part of a single spectrum, and the term REST has been applied.

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