Fetal rhabdomyomatous nephroblastoma. Pathologic histology and special clinical and biologic features.

Fetal rhabdomyomatous nephroblastoma is a very rare variant of Wilms' tumor. The special clinical and histologic features of this variant are presented on the basis of a case seen in a boy who was 13 months old at nephrectomy 8 months ago and has remained healthy since then. The tumor chiefly consists of fetal striated muscle; it occurs predominantly in infancy and early childhood and is often bilateral. Angiography shows a relative paucity of vessels. The volume of the tumor (determined by ultrasonography) decreases only slightly after preoperative radio- and chemotherapy because of a predominance of mesenchymal structures. The finding of nodular renal blastema in renal parenchyma with an apparently normal gross appearance is an argument in favor of classifying this type of tumor in the group of nephroblastomas and may explain the tendency of fetal rhabdomyomatous nephroblastoma to occur bilaterally.