Answer: Mixed epithelial and stromal tumor

Histology: Sections show multiple cysts with intermixed solid areas. The cysts vary from large to microscopic and are lined by columnar to cuboidal epithelium with occasional small papillary tufts. The stroma is variably cellular, is composed of spindle cells with intermixed myxoid stroma and smooth muscle fascicles. No mitosis or atypia is present.

Discussion: Mixed epithelial and stromal tumor (MEST) is more common in women (F:M=6:1), is seen only in adults, is most common in the 5th decade of life and no specific translocation has been described. Other names used to describe this entity are cystic hamartoma and as adult mesoblastic nephroma. The stromal spindle cells are ER and PR positive, the smooth muscle cells are desmin and actin positive, whereas the epithelial cells of the cyst lining are cytokeratin positive.
Morphologically, the differential diagnosis includes multicystic nephroma and cystic renal cell carcinoma. Multicystic nephroma develops in infancy, but may persist into adulthood. They are solitary, well demarcated (usually 5-15 cm) multilocular cystic tumors. The cysts range from 1-3 cm, have thin walls, lack papillary projections and are lined by colloidal epithelium that resemble tubular epithelium. The lining cells may become flattened (in which case it can resemble a lymphangioma) or show hobnail pattern. The stroma is non-descript fibroblastic and may contain smooth muscle, skeletal muscle or cartilage. The spindle cells are ER/PR negative. In cystic renal cell carcinoma the cysts are lined variably atypical cuboidal cells that may have optically clear or granular cytoplasm. The tumor cells may be flattened and on occasion may only be seen focally. The intervening stroma is fibrotic and can be hyalinized and hemorrhagic.