Mixed Epithelial & Stromal Tumor
The tumor is composed of variably-sized cysts that don't communicate with each other or renal pelvis. They are lined by flattened tubular epithelium. The cyst walls have hypercellular spindled stroma with resemblance to ovarian stroma. Frequently, the stromal component expresses estrogen and progesterone receptors indicating the role of steroid hormones in the pathogenesis of this tumor.