Overview

Wilms tumor is a type of childhood cancer that begins in the kidneys. Every person has two kidneys. They are bean-shaped organs located above the waist on both sides of the spine. They are closer to the back of the body than the front. The kidneys filter waste from the blood and make urine. As the kidneys develop in an unborn baby, some of the early cells will become glomeruli (balls of blood vessels that filter water, salt, and waste out of the blood) and some will become nephrons (the tubes through which the water, salt, and waste pass).

If the early cells of the kidney do not develop into glomeruli or nephrons, groups of the immature cells may form in the kidneys when the baby is born. Usually, these cells mature by the time a child is three or four years old, but some may start to grow uncontrollably, forming a mass of immature cells. This mass is called a Wilms tumor.

Wilms tumor is the most common type of kidney cancer in children and is very different from adult kidney cancer. Although Wilms tumor can develop in both kidneys (called bilateral), it usually occurs in only one (unilateral). Rarely, Wilms tumor develops in one kidney first and then the other. Wilms tumor is often found only after it has grown to a size of about eight ounces. (For comparison, a three-year-old child’s kidney usually weighs about two ounces.) About one out of four children with Wilms tumor have evidence that the tumor has spread either to the lung or liver when it is first diagnosed.

Statistics

In the United States, about 500 children are diagnosed with Wilms tumor each year. Wilms tumor occurs most often in the first five years of life and accounts for about 5% of all childhood cancers. The five-year relative survival rate (the percentage of people who survive at least five years after the cancer is detected, excluding those who die from other diseases) of children with Wilms tumor is 92%.

Cancer survival statistics should be interpreted with caution. These estimates are based on data from thousands of cases of this type of cancer, but the actual risk for a particular individual may differ. It is not possible to tell a person how long he or she will live with Wilms tumor. Because the survival statistics are measured in five-year intervals, they may not represent advances made in the treatment or diagnosis of this cancer.

Statistics adapted from the American Cancer Society's publication, Cancer Facts and Figures 2009.

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