Congenital renal tumor: metanephric adenoma, nephrogenic rest, or malignancy?

We report a renal tumor detected by prenatal ultrasound and resected at 2 months of age. This 9-cm, solid mass was composed of tubular and papillary structures lined by small, uniform epithelial cells. There was local invasion into renal parenchyma and a tumor deposit in a hilar lymph node. The tumor was immunopositive for WT1, pankeratin, and CD10; focally positive for CK7; and negative for EMA and TFE3. Based on morphology and immunophenotype, the favored diagnosis was metanephric adenoma over Wilms tumor, renal cell carcinoma, and nephrogenic rest. However, metanephric adenoma only occasionally occurs in children and has never been reported prenatally. Alternatively, this tumor might be a congenital Wilms tumor that differentiated completely. Although the nature of the tumor remains unconfirmed, resection appears to have been curative; the patient remains disease-free 18 months following surgery alone.