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Wilms tumor is a type of cancer that starts in the kidneys. It is the most common type of kidney cancer in children. It is named for Max Wilms, the German doctor who first wrote about it in 1899.

About the kidneys

To understand Wilms tumor, it helps to know something about the kidneys.

The kidneys are 2 bean-shaped organs attached to the back of the belly (abdomen). They are shown in the picture below. 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 kidney's main job is to filter the blood and get rid of extra water, salt, and waste products in the form of urine. Urine leaves the kidneys through long slender tubes called ureters. Each kidney has a single ureter. The ureters empty into the bladder where urine is stored until it is passed out of the body.

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

Although the kidneys are important, people can get by with just one kidney. Today many people are living normal, healthy lives with just one kidney.

Wilms tumors

Wilms tumors are cancers that usually form only in one kidney, but in a small number of cases they form in both. Wilms tumors are often found only after they have grown quite large, but most are found before they have spread to other organs. The average newly-found Wilms tumor is much larger than the kidney it started in.

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

Types of Wilms tumors

There are 2 main types of Wilms tumors. The type depends on how the cells look under a microscope (this is called their histology).

Favorable histology: Cells that look favorable show no anaplasia (see below), although they don't look quite normal. The chance of cure is good. More than 9 out of 10 Wilms tumors have are of this type.

Unfavorable histology: If a tumor looks unfavorable, it means that the centers of the cells are very large and not the right shape. This is called anaplasia. The more anaplasia a tumor has, the harder it is to cure.

In rare cases, children can have other kinds of kidney cancer. The information here is only about Wilms tumor and not other kinds of kidney cancer.

Last Medical Review: 08/01/2012
Last Revised: 08/01/2012