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Wilms tumor is the most common form of kidney cancer in children. It is also called nephroblastoma. "Nephro" means kidney, and a "blastoma" is a tumor made of embryonic tissue that has not yet fully developed. About 500 children are diagnosed with Wilms tumor in the United States each year and about 75% of those patients are under 5 years old.
Approximately 15% of kidney tumors in childhood are not Wilms tumor. To diagnose kidney tumors, doctors need information about the types of cells that make up the tumor. It is important to correctly identify these tumors because they are treated differently from Wilms tumor.
There are many procedures and tests to see if and where cancer cells are present in the body. The exact combination of testing used to diagnose a child depends on the symptoms that are present and the suspected type of cancer.
Typically, cases of Wilms tumor are not diagnosed until the tumor becomes quite large. Fortunately, most tumors are discovered before they metastasize (spread to other organs in the body). As with many cancers, the tumor mass can become quite large; the average weight of a newly discovered tumor is about 500 grams (one pound), which is larger than the normal kidney.
These tests also enable the treating doctors and surgeons to select the best approach for treatment because they show where the tumor is located. The diagnosis is usually confirmed by the surgical removal of all or part of the tumor when a pathologist examines it under a microscope.
In some cases, it is not advisable to perform surgery because it might be dangerous for the child. In these cases, treatment with chemotherapy will be started to shrink the tumor and make removal safer at a later time.
The prognosis (chance of recovery) for children with Wilms tumor is generally very good. Currently, the great majority of children with Wilms tumor are cured. The overall five-year survival rate for children with Wilms tumor is approximately 90%. That means that 9 out of 10 children with Wilms tumor will live at least five years after their cancer is diagnosed. The five-year survival mark is the point at which a patient is considered "cured" since these tumors almost never recur after that time.
There are two major types of Wilms tumors. These are identified based upon the types of cells that are found (histology) and impact the outcome. After the tumor or a portion of the tumor is removed, a pathologist will identify the type of cells that make up the tumor.
Wilms tumor of favorable histologyFavorable histology means there is no anaplasia (distortion of the nuclei of the cancer cells), and that the chance of a cure is excellent. About 92% of Wilms tumors have favorable histology.
This is called “staging” the tumor. Staging allows the cancer care team to identify the best treatment approach using a combination of chemotherapy, surgery and radiation therapy. It will also provide information about the outlook for cure.
The staging system used to describe the extent of spread of Wilms tumors was developed by the National Wilms Tumor Study Group (NWTS) and updated by the Children’s Oncology Group Renal Tumor Committee. The NWTS/COG system describes Wilms tumor stages using Roman numerals I through V (1 to 5).
Stage IThe tumor is only in the kidney, and was removed completely by surgery. The cancer has not grown into blood vessels next to the kidney. The tissue layer (the capsule) that surrounds the cancer was not broken during surgery.
About 40% of all Wilms tumors are stage I.
About 20% of all Wilms tumors are stage II.
About 20% of all Wilms tumors are stage III.
Stage IV The cancer has spread through the bloodstream to other organs far away from the kidneys, such as the lungs, liver, bone or to lymph nodes.
About 10% of all Wilms tumors are stage IV.
Stage VTumors are in both kidneys at the time of diagnosis. Most of the time Stage V tumors are tumors that have developed from primitive, abnormal tissue on both kidneys and do not represent spread from one kidney to the other. The treatment is based on the kidney with the more advanced stage after surgery.
About 5% of all Wilms tumors are stage V.