Renal tumors account for approximately 7% of all childhood cancers. While 80% of these are Wilms tumors (or nephroblastomas), other kidney tumors of childhood include renal cell carcinoma (~4%), clear cell sarcoma of the kidney (~3%), congenital mesoblastic nephroma (~2%), rhabdoid tumor of the kidney (~2%), and cystic nephroma (~2%). Wilms tumor (WT) is the second most common extracranial solid tumor in pediatrics, following neuroblastoma. While overall survival in patients with WT has increased to 90% due to advances in medical and surgical treatment, there remain subsets of patients who do not have favorable outcomes. Moreover, patients with renal cell carcinoma, clear cell carcinoma, and rhabdoid tumor of the kidney have considerably worse outcomes compared to WT. This chapter will focus primarily on WT as the most prevalent pediatric kidney tumor.
Approximately 600 children and adolescents are diagnosed with renal tumors in the United States each year, and of these, about 80% are WTs. The median age at diagnosis of WT is approximately 3.5 years, with some variation based on gender and ethnic group. The majority of WT cases occur prior to 5 years of age, and WT is very rare in children over 10 years of age. Renal cell carcinoma (RCC) is the second most common renal neoplasm in pediatric and adolescent patients, accounting for 2% to 6% of renal malignancies in this age group. While WT is more common in younger children, RCC is most common in ages 15 to 19 years old. Wilms tumor occurs across ethnic and racial groups; the incidence is highest in blacks and lowest in East Asian populations.
In 1899, Max Wilms established the classical description of WT as a mixed tumor, comprised of 3 distinct components: epithelial, blastemal, and stromal. He held that WT cells developed from a common and macroscopically undifferentiated germ cell. This astute discovery highlights that WT arises from disrupted kidney development. In development, the fetal kidney arises from the ureteric bud, which forms collecting ducts, and from the metanephric mesenchyme/blastema, which forms the stroma and remaining tubular structures (glomeruli, proximal and distal tubules, loop of Henle). The blastema usually disappears by 36 week's gestation; however, approximately 1% of infants at birth will retain blastemal components known as nephrogenic rests within kidney tissue. Nephrogenic rests are identified in 40% of patients with WT and 90% of patients with bilateral WT. It is presumed that germline mutations or other inciting events result in nephrogenic rests that in turn transform to WT.
Genetics and Populations at Risk
Most cases of WT are sporadic, though 10% of children with WT have congenital anomalies, either isolated or as part of a genetic syndrome. More than 15 syndromes have been associated with an increased risk of WT (Table 452-1), but those in which the greatest number of WT cases have been ...