Cystic nephroma (CN) and mixed epithelial stromal tumor (MEST) of the kidney have been considered as synonymous terms describing a single nosologic entity in adult patients. Cystic nephroma in pediatric patients (PCN) is, apparently, a completely different nosologic entity. Although the presence of DICER 1 mutations is well established in PCN, nothing is currently known about the DICER 1 gene status in adult MEST/CN. About 33 cases of MEST/CN were selected from the Plzen Tumor Registry; 4 cases were later excluded from the study due to low DNA quality. About 28 of the studied tumors displayed a benign morphology, whereas 1 was diagnosed as a malignant MEST/CN with sarcomatoid differentiation of the stromal component. All 29 samples analyzed using polymerase chain reaction and direct sequencing, including the case with the malignant morphology, were negative for mutation in DICER 1 hot-spot codons 1705, 1709, 1809, 1810, 1813, and 1814. Our results show that MEST/CN has no relation to PCN on a molecular genetic level. On the basis of our findings and the established morphologic differences between PCN and MEST/CN, we conclude that the term CN should be used for pediatric cases only and should be avoided in adult cases of MEST.

Departments of *Pathology †Urology Charles University Medical Faculty and Charles University Hospital Plzen ∥Biomedical Centre Faculty of Medicine in Plzen Charles University Prague Plzen Czech Republic §Department of Pathology Instituto Nacional de Cancerologia Mexico City Mexico ‡Department of Pathology and Laboratory Medicine University of Kansas Medical Center Kansas City KS

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