Diagnosis: Metanephric adenoma.      

            Comments: This tumor is at one end of a pathologic spectrum of lesions and is composed exclusively of epithelial nephroblastic cells; at the other end are tumors composed exclusively of stromal elements (metanephric stromal tumors) and tumors composed of both elements (metanephric adenofibromas). Metanephric adenoma occurs in children and in adults; is well circumscribed but unencapsulated (Fig. 1). Histologically, consists of small cells that form small acini (Fig. 2). May also occur tubular, glomeruloid, and papillary formations the latter with or without calcospherites (Figs. 4 and 5). The differential diagnosis is mainly with papillary renal cell carcinoma, and differentiated epithelial nephroblastoma. Immunohistochemistry is positive for CD57 and negative for CK7 (Fig. 5). The lesions are benign but in rare cases either the epithelial or the stromal component may be associated with the development of malignancy.