What Is Wilms Tumor?

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?

Wilms tumor (also called Wilms’ tumor or nephroblastoma) is a type of cancer that starts in the kidneys. It is the most common type of kidney cancer in children. It is named after Max Wilms, a German doctor who wrote one of the first medical articles about the disease in 1899.

About the kidneys

The kidneys are 2 bean-shaped organs that are attached to the back wall of the abdomen (see picture). One kidney is just to the left and the other just to the right of the backbone. The lower rib cage protects the kidneys.

Small glands called adrenal glands sit on top of each of the kidneys. Each kidney and adrenal gland is surrounded by fat and a thin, fibrous capsule (known as Gerota’s fascia).

illustration showing the kidneys in relation to the renal artery and vein, adrenal gland, ureter, bladder and urethra with a window showing greater detail including adrenal gland, gerota's fascia, renal pelvis, renal artery and vein, ureter and fat

The main job of the kidneys is filtering blood coming in from the renal arteries to rid the body of excess water, salt, and waste products. These substances become urine. Urine leaves the kidneys through long, slender tubes called ureters that connect to the bladder. Urine flows down the ureters into the bladder, and is stored there until the person urinates.

The kidneys also have other jobs:

  • They help control blood pressure by making a hormone called renin.
  • They help make sure the body has enough red blood cells by making a hormone called erythropoietin. This hormone tells the bone marrow to make more red blood cells.

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.

Wilms tumors

Wilms tumors are the most common cancers in children that start in the kidneys. About 9 of 10 kidney cancers in children are Wilms tumors.

Most Wilms tumors are unilateral, which means they affect only one kidney. Most often there is only one tumor, but 5% to 10% of children with Wilms tumors have more than one tumor in the same kidney. About 5% 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.

Even if a doctor thinks a child might have a cancer such as Wilms tumor based on a physical exam or imaging tests, they can’t be sure until a small piece of the tumor is checked in a lab.

Types of Wilms tumor

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

Favorable histology: Although the cancer cells in these tumors don’t look quite normal, there is no anaplasia (see next paragraph). More than 9 of 10 Wilms tumors have a favorable histology. The chance of curing children with these tumors is very good.

Unfavorable histology (anaplastic Wilms tumor): 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. The more anaplasia a tumor has, the harder it is to cure.

Other types of kidney cancers in children

Most kidney cancers in children are Wilms tumors, but 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 unfavorable histology (see Treatment of Wilms tumor 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 tumors 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 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.

The American Cancer Society medical and editorial content team
Our team is made up of doctors and master's-prepared nurses with deep knowledge of cancer care as well as journalists, editors, and translators with extensive experience in medical writing.

Last Medical Review: March 6, 2015 Last Revised: February 16, 2016

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