Wilms tumor (also called Wilms Tumor or nephroblastoma) is a type of childhood cancer that starts in the kidneys. It is the most common type of kidney cancer in children. About 9 of 10 kidney cancers in children are Wilms Tumors.

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Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancerous, and can spread to other areas of the body.