About the kidneys
Every person has two kidneys. They are bean-shaped organs located above the waist on both sides of the spine. They are closer to the back of the body than the front. The kidneys filter waste from the blood and make urine.
As the kidneys develop in an unborn baby, some of the early cells will become glomeruli, which are balls of blood vessels that filter water, salt, and waste out of the blood. Others will become nephrons, which are the tubes through which the water, salt, and waste pass.
About Wilms Tumor
If the early cells of the kidney do not develop into glomeruli or nephrons, clusters of the immature cells may form in the kidneys when the baby is born. Usually, these cells mature by the time a child is three or four years old, but some may grow uncontrollably, forming a mass of immature cells. This mass is called a Wilms tumor. It can also be called a nephroblastoma.
A Wilms tumor is always cancerous and is the most common type of kidney cancer diagnosed in children. A cancerous tumor is malignant, meaning it can grow and spread to other parts of the body. However, a Wilms tumor is very different from adult kidney cancer.
Wilms tumor usually occurs in only one kidney, called unilateral. However, it can develop in both kidneys, called bilateral. Rarely, a Wilms tumor develops in one kidney first and then the other.
A Wilms tumor is often found only after it has grown to a size of about eight ounces—about four times the weight of a healthy three-year-old child’s kidney. About one out of four children with Wilms tumor have evidence that the tumor has spread either to the lung or liver when it is first diagnosed.
Looking for More of an Overview?
If you would like additional introductory information, explore these related items. Please note these links will take you to other sections on Cancer.Net:
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ASCO Answers Fact Sheet: Read a one-page fact sheet (available as a PDF) that offers an easy-to-print introduction to this type of tumor.
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Cancer.Net Patient Education Video: View a short video led by an ASCO expert in childhood cancers that provides basic information and areas of research.
