Association of extrarenal Wilms' tumor with a horseshoe kidney.

Nephroblastoma (Wilms' tumor) is the most common renal malignancy in childhood. Extrarenal Wilms' tumor is uncommon, and the diagnosis is almost always postsurgical. The authors report two cases, located in the sigmoid mesocolon, one of these having an associated horseshoe kidney. Both were treated with excision followed by chemotherapy. They are now 36 months and 7 months postchemotherapy, respectively, and are symptom and recurrence free. Both had favorable histological findings. Association with a horseshoe kidney raises an important issue regarding the histogenesis of these tumors. The authors believe that there is a nexus between the fusion of metanephric blastema during the sixth to seventh week of intrauterine life and the "ectopic" metanephric blastema cells that may give rise to extrarenal Wilms' tumor. Association with a horseshoe kidney with an extrarenal Wilms' tumor has been reported on five previous occasions. The authors closely examine the link between the two.