Answer: Mixed epithelial and stromal tumor

Histology: This lesion is biphasic consisting of cystically dilated space lined by epithelium, which is variably hobnail, atrophic, and cuboidal. The second component of this lesion is a cellular stroma, which in many areas appears fibrous in nature. Both the epithelial and stromal component lack features of malignancy.

Discussion: The lesions seen in this case are classic for mixed epithelial and stromal tumor. These lesions have also been designated as cystic hamartomas of the renal pelvis and adult mesoblastic nephroma. The lesion resembles a multilocular renal cyst in its overall morphology. However, multilocular renal cysts contain multiple cystic spaces similar to what is seen in the current case, yet without the prominent cellular stroma. The septae in between the cysts within multilocular renal cysts typically are delicate, thin, and contain at most a minimal proliferation of variably cellular fibrous tissue. As these two lesions share certain features, there may be a relationship between them. Angiomyolipomas may resemble in terms of their spindle cell component that which is seen within a mixed epithelial and stromal tumor. However, typically within angiomyolipomas the smooth muscle cells will have a more epithelioid appearance and angiomyolipomas lack glandular elements with the possible exception of focally entrapped tubules. The vascular and adipose component seen within angiomyolipomas is lacking within epithelial and stromal renal tumors. Cystic partially differentiated nephroblastomas contain blastema and more primitive tubular components typical of Wilms’ tumor. Mixed epithelial and stromal tumors of the kidney are almost uniformly seen in women. The lesions tend to be well-circumscribed masses with both solid and cystic components. A characteristic finding seen in some cases is the projection of this lesion into the renal pelvis and proximal ureter. These lesions are benign and express various muscle markers yet are negative for HMB45 and CD34.