Multifocal metanephric adenoma in childhood.

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.