Comments: This photomicrograph is from a congenital mesoblastic nephroma that presented as a 5.1 cm right renal mass in a newborn female at 33 weeks gestation. It is the most common congenital renal neoplasm and comprises about 2% of all pediatric renal tumors. Congenital mesoblastic nephroma, classic type, has a whorled cut surface similar to leiomyomas. The tumor consists of intersecting bundles of fibroblastic cells invading the renal parenchyma.
Comments: The classic variant of congenital mesoblastic nephroma resembles benign fibromatosis (as seen here). The cellular variant consists of ill-defined bundles of mitotically active plump epithelioid cells. Occasional tumors may have features of both variants.
Comments: At higher magnification, mitotic figures were easily found in this congenital mesoblastic nephroma. Simple nephrectomy is usually curative for this tumor; recurrences and metastases are uncommon.
