Metanephric adenoma: histology, immunophenotype, cytogenetics, ultrastructure.

Metanephric adenoma is a rare renal neoplasm that is histologically and clinically unique. We found this neoplasm in a 54-year-old woman. This large, well-circumscribed, solid, tan tumor showed histologic similarity to developing metanephric tubular epithelium. It is composed of uniformly small epithelial cells forming tubules. Immunohistochemical evaluation showed that the tumor cells express proximal tubule marker URO-2 and wt1 gene protein product, commonly expressed in Wilms' tumors. Ultrastructurally, the epithelial cells were characterized by the presence of cilia on the luminal side and were resting on an abundant basement membrane. Cytogenetic analysis showed normal female karyotype. The unique features of metanephric adenoma should be clinically and pathologically recognized because of its invariably benign course.