Renal epithelial and stromal tumor: a new term to encompass the spectrum of cystic nephroma and mixed epithelial and stromal tumor of the kidney

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Cystic nephroma (CN) and mixed epithelial and stromal tumor of the kidney (MEST) are rare benign cystic renal neoplasms, which have been the subject of considerable recent controversy. Within the past 2 years several studies have emphasized the overlapping clinical, morphologic and immunohistochemical features between these tumors, including their predominance in middle-aged to older women, a variably cystic architecture with flat to hobnail cells lining the cystic spaces and presence of ovarian-type stroma with occasional inhibin-positive luteinized cells. Thus, we have proposed the unifying term of ‘renal epithelial and stromal tumor’ (REST) to convey the idea that CN and MEST most likely represent ends of the spectrum of the same entity.