Mesenchymal renal tumors in infancy: a reappraisal.

Eight cases of primary mesenchymal renal tumor in infants under one year of age were studied. The patients' ages ranged from one day to seven months; all but one were diagnosed within the first three months of life. There were four typical congenital mesoblastic nephromas. Two malignant mesenchymal, one intermediate case of difficult classification designated as "cellular variant" of congenital mesoblastic nephroma, and one hemangioendothelioma. All patients were alive and free of tumor 3 9/12 to 18 10/12 years after surgery. Of the four infants with congenital mesoblastic nephroma, one was treated by tumor excision alone; nearly 13 years later this patient was free of tumor. A high degree of cellularity and a high nucleus-cytoplasm ratio were features that characterized the tumor diagnosed as "cellular variant" of congenital mesoblastic nephroma. A sarcomatous gross appearance with cavitation necrosis was seen in the two instances of malignant mesenchymal nephroma; one of these metastasized to the lung, whereas in the other, though no metastases developed, all the histologic details of the previous case were reproduced, including distinct foci of necrosis and a high mitosis rate. Mesenchymal renal tumors in young infants constitute a set more heterogeneous than has been previously assumed. They should not be considered uniformly benign. As a group, they span the whole spectrum between benign, morphologically quiescent lesions, clinically and pathologically intermediate or indeterminate ones, and outright malignant tumors with a high risk of distant spread.