Wilms' tumor is one of the most common tumors of the abdomen in children and the most common type of kidney tumor. The exact cause of tumor formation in most children is unknown. It is associated with certain birth defects including urinary tract abnormalities, absence of the iris (aniridia), and hemihypertrophy (enlargement of one side of the body). It is more common among some siblings and twins, which suggests a possible genetic cause. The tumor may become quite large, but usually remains encapsulated (self-enclosed). It may spread to other body tissues, especially the lungs.
The disease is estimated to occur in about 1 out of 200,000 to 250,000 children. The peak time of occurrence is at 3 years old, and Wilms' tumor is rare after the age of 8 years.
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