Intrapelvic Wilms tumor: report of 2 cases and review of the literature.

Wilms tumor arising from the renal parenchyma usually presents initially as an abdominal mass. A review of the literature revealed fewer than 10 cases manifesting as a renal pelvic mass occupying primarily the collecting system. We have treated 2 patients with this unusual presentation: 1) a 4-month-old male infant whose initial radiological investigation demonstrated a left renal neoplasm measuring 10 x 7 x 5 cm.; bisection of the kidney showed a tumor completely filling the collecting system to the proximal ureter with extreme attenuation of the renal parenchyma, and 2) a 9-year-old boy who was hospitalized due to hematuria; ultrasonography, computerized tomography and magnetic resonance imaging showed a mass in the right renal pelvis; gross pathology displayed a 7 x 6 x 5 cm. polypoid mass occupying the renal pelvis without parenchymal involvement. Microscopically, both lesions were typical Wilms tumors. We discuss the characteristic clinical manifestations, diagnosis and treatment, and review the literature.