Spontaneous Renal Mixed Epithelial and Stromal Tumor (MEST) in a Beagle

A mixed epithelial and stromal tumor (MEST) of the kidney was found in a 22-month-old female Marshall beagle. Histopathologically, this lesion was well demarcated and was composed of a mixture of epithelial and stromal elements. The epithelial elements, which contained columnar or ciliated columnar epithelia, had cystic and tubular growth patterns. The tubular lumina contained eosinophilic materials, which were positive for periodic acid Schiff and alcian blue (pH 2.5). Immunohistochemically, the epithelia were positively stained with proliferative cell nuclear antigen (PCNA) and showed co-expression of cytokeratin and vimentin. Morphologically, these proliferative ductal structures were suggestive of a primitive duct arising from the mesonephric or paramesonephric tubule. The stromal elements were characterized by proliferation of spindle cells embedded in a collagen-rich eosinophilic matrix. Additionally, stromal cells were positive for PCNA, vimentin and muscle markers (α-smooth muscle actin and desmin). Therefore, this lesion was ultimately diagnosed as a MEST of the kidney.