Wilms' tumor, also called nephroblastoma, is the most common form of kidney cancer in children. It develops when cancerous (malignant) kidney cells multiply out of control, eventually forming a mass. This mass usually is smooth and fairly round.
As it grows, Wilms' tumor changes the normal shape and appearance of the kidney. It also can destroy normal kidney tissue and cause bleeding into the urine. In some cases, the tumor eventually becomes noticeable as a firm, smooth lump in the child's side or abdomen. Without proper treatment, the cancer can spread to other organs—usually the lungs and liver.
Wilms' tumor is uncommon, accounting for just a small percentage of all childhood cancers. It is much more common in African American children than in white or Asian American children, and it is slightly more common in girls than in boys. In most cases, the tumor affects only part of one kidney. But sometimes, tumors affect both kidneys or occur in multiple parts of a kidney.
Specific genetic mutations have been identified in some patients with Wilms' tumors; having these mutations may mean a less favorable prognosis. Testing for these mutations can help doctors determine the best treatment.
Children with certain birth defects are more likely to develop Wilms' tumor, too. These include not having irises in the eyes and excessive growth on one side of the body.