Wilms' tumor is an embryonal (primitive cells similar to those seen in the embryo) tumor of renal (kidney) origin and the most common kidney tumor in children. Approximately 400 cases occur each year in the United States. It is more common in African American children. Its highest incidence is between 2-5 years of age. It is one of the most curable of all childhood tumors. Associated anomalies are common. A multimodality approach utilizing surgery, radiation, and chemotherapy are routinely used in treatment.

Anatomy and Physiology

• The kidneys are a pair of bean shaped reddish-brown organs that lie on either side of the spinal column and just below the diaphragm. They are about 5 inches (12.5 cm.) long and 3 inches (7.5 cm.) wide (Figure 1)
  
• On the medial (facing the spine) border the kidney is notched at the hilus, the point where the major artery to the kidney enters and vein leaves
  
• The ureter also leaves from the hilus
  
• The kidney is made up of over a million renal tubules (nephrons). All the nephrons together form the cortex . The nephrons filter the blood of waste products that pass into the urine
  
• The urine passes from the nephrons into collecting tubes called calyxes and then into the renal pelvis (the dilated upper portion of the ureter) and into the ureter, which conducts the urine into the urinary bladder
  
  1. The wall of the renal pelvis and ureter are composed three layers. The inner layer called the mucosa is lined with cells called transitional cells
     
  2. The middle layer is composed of muscle
     
  3. The outer layer is composed of fibrous tissue and is called the adventitia
     
• Two kidneys are not necessary for survival. In fact, less than one kidney is all that is necessary
  
• On the upper surface of each kidney lies the adrenal gland, a small pyramid shaped tissue that produces steroid hormones and adrenaline
  
• A capsule of thin tissue encloses each kidney
  
• The renal fascia (Gerota's fascia) is a membrane layer that encloses the kidney and adrenal gland. It is separated from the capsule of the kidney by the perinephric space, which contains fat

Figure 1 - Anatomy of the kidney. The upper portion of the kidney is cut away to expose the cortex, calyxes and renal pelvis. The renal artery and ureter enter and the renal vein leaves the kidney at the hilus. The adrenal gland rests on the upper pole of the kidney. © N. Gordon