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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.

Cancer starts when cells in the body begin to grow out of control. Cells in nearly any part of the body can become cancer, and can spread to other areas of the body. To learn more about how cancers start and spread, see What Is Cancer? For information about the differences between childhood cancers and adult cancers, see Cancer in Children.

About the kidneys

The kidneys are 2 bean-shaped organs that are attached to the back wall of the abdomen, just to the left and right of the backbone. The kidneys do a number of things:

• They filter the blood to remove excess water, salt, and waste products, which leave the body as urine.
• They help control blood pressure.
• They help make sure the body has enough red blood cells.

On top of each kidney is a small gland called an adrenal gland. The adrenal glands make hormones that have many functions, including helping the body burn fat and protein and respond to stress.

Each kidney and adrenal gland is surrounded by fat and a thin, fibrous capsule (known as Gerota’s fascia).They are protected by the lower rib cage.

Our kidneys are important, but we actually need less than one complete kidney to do all of its basic functions. Many people in the United States live normal, healthy lives with just one kidney.

Most Wilms tumors are unilateral, which means they affect only one kidney. Most often there is only one tumor, but a small number of children with Wilms tumors have more than one tumor in the same kidney. About 5% to 10% of children with Wilms tumors have bilateral disease (tumors in both kidneys).

Wilms tumors often become quite large before they are noticed. The average newly found Wilms tumor is many times larger than the kidney in which it started. Most Wilms tumors are found before they have spread (metastasized) to other organs.

Types of Wilms tumor

Wilms tumors are grouped into 2 major types based on how they look under a microscope (their histology):

• Favorable histology: The cancer cells in these tumors don’t look quite normal, but there is no anaplasia (see next paragraph). About 9 of 10 Wilms tumors have a favorable histology. The chance of curing children with these tumors is very good.
• Anaplastic histology: In these tumors, the look of the cancer cells varies widely, and the cells’ nuclei (the central parts that contain the DNA) tend to be very large and distorted. This is called anaplasia. In general, tumors in which the anaplasia is spread throughout the tumor (known as diffuse anaplasia) are harder to treat than tumors in which the anaplasia is limited just to certain parts of the tumor (known as focal anaplasia).

Other types of kidney cancers in children

In rare cases, children can develop other types of kidney tumors.

Mesoblastic nephroma

These tumors usually appear in the first few months of life. Children are usually cured with surgery, but sometimes chemotherapy is given as well. These tumors sometimes come back soon after treatment, so children who have had these tumors need to be watched closely for the first year afterward.

Clear cell sarcoma of kidney (CCSK)

These tumors are much more likely to spread to other parts of the body than Wilms tumors, and they are harder to cure. Because these tumors are rare, treatment is often given as part of a clinical trial. It's usually much like the intensive treatment used for Wilms tumors with anaplastic histology (see Treatment of Wilms Tumors by Type and Stage).

Malignant rhabdoid tumor of the kidney

These tumors occur most often in infants and toddlers. They tend to spread to other parts of the body quickly, and most have already spread by the time they are found, which makes them hard to cure. Because these tumors are rare, treatment is often given as part of a clinical trial, and usually includes chemotherapy with several different drugs.

Renal cell carcinoma

This is the most common type of kidney cancer in adults, but it also accounts for a small number of kidney cancers in children. It’s rare in young children, but it’s actually more common than Wilms tumor in older teens.

Surgery to remove the kidney (or just the tumor) is the main treatment for these cancers if it can be done. The outlook for these cancers depends largely on the extent (stage) of the cancer at the time it’s found, whether it can be removed completely with surgery, and its subtype (based on how the cancer cells look under a microscope). If the cancer is too advanced to be removed by surgery, other types of treatment may be needed.