Wilms' tumor is the most common primary renal malignancy and intra-abdominal tumor of childhood. Ninety per cent occur in children less than 7 years of age, with a median age of 3-1/2 years. The children are usually asymptomatic, most often presenting with clinical suspicion of an abdominal mass (approximately 70%). Other symptoms include abdominal pain (30%), hematuria (15 to 20%), and less commonly with moderately severe hypertension and nonspecific constitutional symptoms (fever, malaise, gastrointestinal complaints). There is a higher incidence of Wilms' tumor in patients with certain congenital abnormalities such as aniridia, hemi-hypertrophy, Beckwith-Wiedemann syndrome, nephroblastomatosis, and various genitourinary abnormalities (hypoplasia, fusion, ectopia, duplicated collecting systems, hypospadias, and cryptorchidism).

Wilms' tumor is secondary to proliferation of metanephric blastema. It is typically bulky and usually extends within the renal parenchyma, displacing and distorting the pelvocalyceal system. However, the mass may be exophytic or may, although rarely, be extrarenal. The outcome following multimodal therapy depends on the initial stage and whether or not the tumor has favorable or unfavorable histology for response to treatment. The unfavorable types include anaplastic, sarcomatous, and rhabdoid types.

The following stages have been described for Wilms' tumor:

I: Tumor limited to kidney and completely resected, negative margins, intact renal capsule, not rupture.

II: Tumor extends beyond kidney, but completely excised, negative margins, regional infiltration, spillage.

III: Residual nonhematogenous tumor, confined to abdomen including lymph nodes, peritoneal implants, incomplete excision.

IV: Hematogenous metastases beyond stage III, i.e., lung, liver, bone, brain.

V: Bilateral renal involvement, at time of diagnosis.

Wilms' tumor is bilateral (Stage V) in 5 to 10% at time of initial diagnosis. In a patient with unilateral Wilms', a contralateral tumor may develop 5 to 10 years later, although the majority occur within 2 years. The work-up should include a plain abdominal supine radiograph, not only to look for mass affect but also to look for calcifications, skeletal abnormalities, and paravertebral masses which may help in differentiating Wilms' tumor from neuroblastoma.

The next imaging study should be abdominal and pelvic sonography to determine the organ of origin, the size and extent of the mass, the characteristics of the mass (solid, cystic, complex calcifications), the presence of regional lymphadenopathy, liver metastases, tumor thrombus in the renal vein, inferior vena cava, and/or right atrium, and to look for bilaterality of the mass. One should also look for any evidence of nephroblastomatosis which appears sonographically as hypoechoic masses in the periphery of the kidney, thickening of the renal capsule, or as nodularity or lobulation of the kidney. This entity, which may be an anlage for Wilms' tumor, is much more easily seen with contrast- enhanced CT. Duplex and/or color Doppler imaging are particularly helpful in the evaluation of the venous vasculature structures that may be involved with Wilms' tumor. Wilms' tumor is most often a solid mass, although cystic spaces may often be seen within the mass due to internal hemorrhage and/or necrosis. We have noted that some of the cystic spaces within Wilms' tumor are large venous channels due to arteriovenous shunting.

Depending upon the sophistication of the ultrasound, one may choose to also obtain a baseline CT, unless nephroblastomatosis is suspected. Certainly a baseline chest CT is recommended prior to surgery to look for pulmonary metastases. During and after their treatment, sonography is an excellent noninvasive means of following the children.