What is Wilms tumor?
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 that are attached 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' main job is to filter blood and rid the body of excess water, salt, and waste products. The filtered products and extra water are changed into 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 help control blood pressure by making a hormone called renin.
The kidneys 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 one 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 are cancers that can start anywhere in the kidneys. 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 (cancer 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 developed. Most tumors are found before they have spread (metastasized) 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: Although the cells in the tumor 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 cure for children with these tumors is very good.
Unfavorable histology (anaplastic Wilms tumor): 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 tumors in children
About 9 of 10 kidney tumors that occur in children are Wilms tumors, but in rare cases children may develop other types of kidney tumors.
These tumors usually appear in the first few months of life. Patients are usually cured with surgery, but sometimes chemotherapy is given as well. Children who have had these tumors need to be watched closely for the first year after treatment.
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 is usually similar to the intensive treatment used for Wilms tumors with unfavorable histology (see “Treatment of Wilms tumor by type and stage”).
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 is rare in young children, but it is actually more common than Wilms tumor in older teens. The treatment and outlook for these cancers depends largely on the extent (stage) of the cancer at the time it is found, whether it can be completely removed with surgery, and its subtype (based on how the cancer cells look under a microscope). If it is more advanced, other treatments may be needed.
The rest of this document refers only to Wilms tumor.
Last Medical Review: 07/27/2012
Last Revised: 01/17/2013