Teratoid Wilms tumor arising as a botryoid growth within a supernumerary ectopic ureteropelvic structure

We report a case of a teratoid Wilms tumor arising within a supernumerary ectopic ureteropelvic structure in a 7-year-old boy. The tumor was near the right kidney but was completely separate from it. On the contralateral side, the child had a duplication of the ureteral system. Pathologic examination revealed the mass to be completely enveloped by a fibromuscular sac lined by urothelium. The tumor had a botryoid, polypoid architecture and, in addition to areas of classic Wilms tumor, had extensive squamous, mucinous, and columnar elements. Teratoid Wilms tumors are rare and, to the best of our knowledge, have not previously been described as arising either in anomalous urinary structures or at 7 years of age.