Cystic nephroma (multilocular cyst) and mixed epithelial and stromal tumor of the kidney: a spectrum of the same entity?

The recently described mixed epithelial and stromal tumor (MEST) of the kidney and adult cystic nephroma (CN) (multilocular cyst) are rare benign cystic renal neoplasms that are composed of epithelial and stromal elements. Consensus criteria for distinguishing these entities have not been well established. Our objective in this study was to evaluate cases of CN and MEST to define the morphological, immunophenotypic, and clinical features of these two entities. Eleven cases from the files of a single institution diagnosed from 1996 to the present as either CN or MEST were reviewed. Architecturally, all lesions were composed of multiple noncommunicating cysts lined by a single layer of epithelial cells. All cases had areas with increased stromal cellularity and 8 cases had ovarian-like stroma present at least focally within the tumor. No stromal or epithelial cell atypia, blastemal elements, or increased mitotic activity was appreciated. The immunoprofile was also similar in the 7 cases stained and included epithelial reactivity with keratin and CAM 5.2 and stromal reactivity with estrogen receptor, progesterone receptor, smooth muscle actin, WT-1, vimentin, and focal desmin. All cases have acted in a benign fashion with no history of recurrence or metastasis. We propose that CN and MEST of the kidney represent a spectrum of the same entity. If the diagnosis of CN is limited to cases that are comprised entirely of thin fibrous-walled cysts, all 11 of our cases would be classified as MEST.