What Is It?
Wilms' tumor, also called nephroblastoma, is the most common form of kidney cancer in children. It develops when cancerous (malignant) kidney cells multiply out of control, eventually forming a mass. This mass usually is smooth and fairly round.
As it grows, Wilms' tumor changes the normal shape and appearance of the kidney. It also can destroy normal kidney tissue and cause bleeding into the urine. In some cases, the tumor eventually becomes noticeable as a firm, smooth lump in the child's side or abdomen. Without proper treatment, the cancer can spread to other organs—usually the lungs and liver.