Metanephric
adenoma is the most commonly occurring member of the metanephric
tumor family, which also includes metanephric
adenofibroma and metanephric stromal
tumor. According to the World Health Organization classification, however, it is not commonly multifocal. Reported herein is the case of a 9-year-old boy with multifocal metanephric
adenoma. Histologically, surgical sections showed multifocal proliferation of small rounded and uniform cells with smooth nuclear contours, scant pale-staining cytoplasm, dark-staining nuclei, and inconspicuous nucleoli: the cells were arranged in sheets and acinal, ductal, glomeruloid, and papillary structures. On immunohistochemistry the
tumor cells were positive for
vimentin, cytokeratins (
CAM5.2, AE1/AE3, and CK18), and WT1, but negative for
cytokeratin 7 (CK7) and
epithelial membrane antigen (EMA). The Ki-67 labeling index was <1%. In addition, cytogenetic analysis indicated a normal karyotype (46XY). Other histologically similar
tumors are
papillary renal cell carcinoma and
nephroblastoma, and it is necessary to distinguish metanephric
adenoma from those
tumors because of
malignancy. In contrast to those
tumors, metanephric
adenoma has inconspicuous nucleoli, loss of CK7 and EMA expression, and no mitotic figures. Thus, the histological and immunohistochemical features of the present case were compatible with metanephric
adenoma.