Chapter 88

### Key Points

1. Wilms tumor is the second most common abdominal tumor in children and accounts for 95% of the renal tumors in children.

2. The overall survival for all patients with Wilms tumor is >90%.

3. For unilateral tumors, upfront resection with lymph node biopsy is the current recommendation from the Children's Oncology Group.

4. Children with bilateral renal masses require no biopsy; they are begun on 3-drug chemotherapy for 6 to 12 weeks prior to local control with nephron-sparing surgery.

5. Patients are treated on risk-based protocols with molecular risk factors being integrated into the assessment that includes stage and histology.

### Pathophysiology

#### Epidemiology

Wilms tumor is the second most common intra-abdominal cancer of childhood and the fifth most common pediatric malignancy overall. It represents approximately 6% of all pediatric cancers and accounts for more than 95% of all tumors of the kidney in the pediatric age group. In the United States, there is an annual incidence of 8 cases per million children younger than 15 years of age, with the total incidence estimated at about 500 cases per year. Approximately 75% of the cases occur in children younger than 5 years of age, with a peak incidence at 2 to 3 years of age. Because of the rarity of this tumor, clinical investigation conducted in an organized manner was begun in 1969 through the establishment of the National Wilms Tumor Study Group (NWTSG). This represented a cooperative effort among several groups to treat patients in a clearly defined manner so that statistically relevant comparisons of treatment variations could be made. Due, in large part to this effort, survival for patients with Wilms tumor, when considered as a whole, once less than 30%, is currently greater than 90%. In 2001, the NWTSG was incorporated into the larger Children's Oncology Group (COG).

#### Pathology

Classic Wilms tumor has a triphasic appearance, with the 3 cell types being stromal, epithelial, and blastemal. All 3 elements are not required, however, to have a diagnosis of Wilms tumor. The neoplastic cells can often be seen to be forming primitive tubules and glomeruli. One of the major contributions of the NWTS was a report by Beckwith and Palmer that separated Wilms tumors into distinct histopathologic categories based on prognosis. An analysis of 427 specimens found that 11% of the total patients in this study contributed to 52% of the mortality. Since that study, Wilms tumors have been classified into favorable and unfavorable histologic groups. The unfavorable histologic group includes Wilms tumors with anaplasia and 2 distinct renal tumors: clear-cell sarcoma of the kidney (CCSK) and malignant rhabdoid tumor of the kidney. Anaplasia can be focal or diffuse in nature, with the focal subtype being somewhat more favorable, and is characterized by cells with large, polyploid nuclei and multipolar mitotic figures. Anaplasia was present in 4.5% of cases entered on NWTS-III and is more common in ...

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