Fetal rhabdomyomatous nephroblastoma with a renal pelvic mass simulating sarcoma botryoides.

Fetal rhabdomyomatous nephroblastomas are very rare histologic variants of Wilms' tumor. These neoplasms are composed predominantly of fetal striated muscle and isolated regions of typical trimorphic nephroblastoma. This case is presented because in addition to its rarity, part of the tumor formed a renal pelvic mass which simulated sarcoma botryoides. However, ultrastructural analysis of this area revealed nephronogenic epithelial differentiation rather than embryonal rhabdomyoblasts, a distinction of important clinical significance. The literature is reviewed, and the clinicopathologic importance of distinguishing this variant from classic Wilms' tumor, the rhabdomyosarcomatous type of Wilms' tumor and congenital mesoblastic nephroma is stressed.