Wilms tumor is the second most common intra-abdominal cancer of childhood and the fifth most common pediatric malignancy overall. It accounts for approximately 6% of all pediatric cancers and for more than 95% of all tumors of the kidney in the pediatric age group. In the United States there are approximately eight cases of Wilms tumor per million children less than 15 years of age per year, with the total number of new cases being estimated at about 500 cases per year. Approximately 75% of the cases occur in children less than 5 years of age with a peak incidence at 2 to 3 years of age. Survival for patients with Wilms tumor, when considered as a whole, once <30%, is currently greater than 90%, making it one of the successes of modern medicine. This dramatic improvement in survival is due, in part, to the systematic manner in which the approach to therapy has evolved. Surgery is a critical component of this therapy and the role of the surgeon is central in the treatment of children with Wilms tumor. However, the addition and refinement of chemotherapy, and, in certain circumstances, radiation therapy, have also had a significant impact on achieving improved survival rates. As with most pediatric cancers, treatment for Wilms tumor is now based on risk stratification that incorporates not only stage and histology but also genetic markers. Current therapy has been refined so that children at low risk of tumor recurrence are spared intensive chemotherapy and radiation, with their long-term side effects, without sacrificing excellent outcomes. Conversely, research on novel treatment strategies continues for children with tumors at high risk of recurrence.

The vast majority of Wilms tumors occur in one or the other kidney—no side predilection has been observed. However, extrarenal Wilms tumor can sometimes occur. In addition, approximately 5% to 10% of children with Wilms tumor present with disease in both kidneys (synchronous, bilateral Wilms tumor). Variations in renal anatomy can occur. It is critical to recognize certain variations, such as the occurrence of Wilms tumor in a solitary or horseshoe kidney, as they significantly alter the treatment approach. Variations in the renal vasculature are fairly common. These include multiple renal arteries and/or veins, and a circumaortic or retroaortic left renal vein. However, these variants in the renal vasculature rarely impact the conduct of a radical nephrectomy and do not need to be exhaustively sought with preoperative imaging. Variations in the anatomy of the collecting system can also occur, most commonly duplication of the ureter. As with the variations of renal vasculature, however, variations of the collecting system should not significantly alter the conduct of surgery.

Children with Wilms tumor typically present with an asymptomatic abdominal mass. It is not uncommon for the tumor to be discovered by a parent while bathing the child or a relative who notices a protuberant abdomen. Associated signs and symptoms such as malaise, pain, and either microscopic or gross hematuria are found in approximately 20% to 30% of the children. Hypertension, presumably due to increased renin activity, is present in approximately 25% of children with Wilms tumor. Occasionally a child will present with a rapidly enlarging abdominal mass, anemia, hypertension, pain, and fever. These children usually have a subcapsular hemorrhage within the tumor that leads to these symptoms.

Although most cases of Wilms tumor are sporadic, others occur as part of a syndrome and may, therefore, be detected as part of a screening evaluation. Beckwith–Wiedemann syndrome is one where Wilms tumor is a common component. This syndrome consists of a number of abnormalities that include macroglossia, macrosomia, hypoglycemia, visceromegaly, and omphalocele, in addition to a predisposition to a number of tumors, most commonly Wilms tumor. Patients with the rare congenital WAGR syndrome have Wilms tumor, aniridia, genitourinary malformations, and mental retardation. Another syndrome associated with Wilms tumor is the Denys–Drash syndrome. In this syndrome patients have severe genitourinary abnormalities (e.g., male pseudohermaphroditism) and renal failure secondary to progressive, diffuse glomerular nephropathy. Of these patients, 50% to 90% will develop Wilms tumor.

The main conditions in the differential diagnosis in a young child presenting with an abdominal mass are Wilms tumor and neuroblastoma, although other solid tumors of childhood, such as hepatoblastoma, germ cell tumor, or rhabdomyosarcoma, should also be included. The distinction between Wilms tumor and neuroblastoma is usually relatively straightforward because Wilms tumor is intrarenal with a characteristic intrinsic abnormality of the urinary collecting system seen on imaging studies. Neuroblastoma arises within the adrenal gland or the paravertebral sympathetic ganglia. Tumors that cross the midline are more likely to be neuroblastoma, although very large Wilms tumors can present as midline abdominal masses. Conditions such as multicystic kidneys and obstructive uropathy should also be considered in the differential diagnosis of an abdominal mass in a young child. Wilms tumor presenting as an abdominal mass on the left side must be distinguished from an enlarged spleen.

The workup of a child with an intra-abdominal mass suspected of being a Wilms tumor should proceed in a systematic fashion. Real-time ultrasonography is usually the initial study and can determine whether or not the mass is intrarenal or extrarenal, and also whether the lesion is cystic or solid in consistency. Computed tomography (CT) of the abdomen and pelvis is generally the imaging study of choice for patients suspected of having a renal tumor. This will confirm the presence of a solid renal mass and will also afford the opportunity to visualize the contralateral kidney to confirm its presence (and function), and to exclude synchronous bilateral disease with a high degree of sensitivity.

Intravascular tumor extension into the inferior vena cava (IVC) occurs in about 6% of Wilms tumor cases. Therefore, this should be specifically investigated in the preoperative evaluation of all children with a renal mass, as it may alter the conduct of surgery. This can generally be done most easily and accurately with Doppler ultrasonography. Magnetic resonance imaging (MRI) can also be used to assess intravascular tumor
extension but usually requires sedation in young children, and so is not routinely used. However, it may be helpful in defining an extensive tumor thrombus that extends up to the level of the hepatic veins or even into the right atrium. In addition, MRI may be useful in distinguishing Wilms tumor from nephrogenic rests (NRs) (see below). Finally, echocardiography may be useful in rare circumstances to demonstrate (or exclude) intracardiac tumor extension.

The most common site of metastatic spread of Wilms tumor is the lungs; chest CT is the preferred imaging modality for evaluating this site. Other rare sites of metastases, such as the liver, are usually well evaluated with the initial abdominopelvic CT scan.

Approximately 25% of children with Wilms tumor will present with hypertension. Therefore, a preoperative assessment of blood pressure should be performed in all patients, with hypertension being medically controlled prior to surgery. In many patients, but not all, the hypertension will resolve after radical nephrectomy, although it may take some time. Patients with Wilms tumor rarely present with renal insufficiency and so formal preoperative assessment of renal function is usually not required. Although patients with a Wilms tumor predisposition syndrome may have intrinsic renal disease, it is usually not of clinical significance until a few years later. Appropriate genetic testing may be done, as indicated. Children with Wilms tumor will occasionally present with pulmonary insufficiency due to extensive metastatic disease in the lungs. The clinical assessment of a patient is generally sufficient to determine whether they can tolerate general anesthesia; formal pulmonary function studies are not required. As Wilms tumor is generally very chemosenstive, the tumor burden in the lungs and the functional pulmonary status should improve fairly promptly once chemotherapy has been initiated.