Cytodifferentiated renal tumors occurring with Wilms' tumors of the opposite kidneys: report of two cases.

The most common differentiated renal tumor of early childhood is the congenital mesoblastic neophroma. Well-differentiated forms of Wilms' tumor may present a difficult differential diagnosis from these benign neoplasms. The cases of two patients found to have well-differentiated renal neoplasms after being treated for Wilms' tumor of the opposite kidneys are reported. The second neoplasms were composed of a benign-appearing stroma with mature skeletal muscle and completely lacked any embryonal, nephroblastic tissue. These benign-appearing neoplasms may be cytodifferentiated variants of Wilms' tumor. Though tumors of this type may have a malignant potential, there is evidence to indicate that they may be approached more conservatively than the usual nephroblastoma. In cases of bilateral Wilms' tumor, well-differentiated tumors might be treated by partial nephrectomy alone, with careful preservation of functional renal tissue.