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

To understand Wilms tumor, it helps to know about the normal structure and function of the kidneys.

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

The kidneys help control blood pressure.  They also help make sure the body has enough red blood cells. They do this by making a hormone called erythropoietin, which tells the bone marrow to make more red blood cells.

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

Wilms tumors

Wilms tumors may start anywhere in the kidneys. Most Wilms tumors are unilateral, which means they affect only one kidney. In most cases there is only one tumor, but in 5% to 10% of cases there is more than one tumor in the same kidney. About 5% of children with Wilms tumor have bilateral disease (cancer in both kidneys).

Wilms tumors often become quite large before they are noticed. The average weight of a newly found Wilms tumor is about 1 pound -- many times larger than the kidney in which it developed. Most tumors are found before they have spread (metastasize) to other organs.

Even though doctors may think a child has a cancer such as Wilms tumor based on a physical exam or imaging tests, they cannot be certain until a sample of the tumor is looked at under a microscope.

Types of Wilms tumor

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

Favorable histology: These Wilms tumor have a favorable appearance under the microscope. Although the cells aren't quite normal looking, there is no anaplasia (see next paragraph). More than 90% of Wilms tumors have a favorable histology. The chance of cure for these tumors is very good.

Unfavorable histology (anaplastic Wilms tumor): These Wilms tumors have an unfavorable appearance under the microscope. The look of the cancer cells varies widely, and the cell's nuclei (the central parts that contains the DNA) tend to be very large and distorted. This is called anaplasia. The more anaplasia that is found, the poorer the chance is for a cure.

Other Types of Kidney Tumors in Children

About 9 out of 10 of kidney tumors that occur in children are Wilms tumors. Very rarely, children may develop other types of kidney tumors.

Mesoblastic nephroma

This tumor usually appears in the first few months of life. These tumors are usually curable with surgery.

Clear cell sarcoma of kidney (CCSK)

These tumors are much more likely to spread than Wilms tumors and are harder to cure. Treatment is usually similar to the intensive treatment used for unfavorable histology Wilms tumor.

Rhabdoid tumor of the kidney

These tumors occur most often in infants. 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. Treatment is usually similar to the intensive treatment used for unfavorable histology Wilms tumor.

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.

The rest of this document refers only to Wilms tumor.

Last Medical Review: 09/14/2009
Last Revised: 09/14/2009