Childhood Kidney Cancer
Wilms (or ‘nephroblastoma’) is the most common type of kidney cancer in children. About 1 in 10,000 diagnosed with the disease each year in North America. Overall around 7,5000 children under the age of 18 suffer from Wilms at any given time and about 250,000 worldwide.
It begins in the womb with the tumour growing on the kidney with the most common symptom being a sudden swollen stomach. Eventually the tumor becomes much bigger than the kidney itself and both usually require removal. Later stage Wilms can spread to other parts of the body including the lungs, liver and bones.
Wilms’ tumours usually affect children under the age of five.
Most children affected with Wilms tumor have no clear underlying cause for their disease.
Several small studies have suggested that certain environmental factors may be associated with a higher risk of Wilms tumour, but the results were conflicting.
A few children are born with a genetic predisposition to Wilms tumor, such as children with the following factors:
Congenital abnormalities: It is well documented that certain syndromes are associated with Wilms tumor. The syndromes associated with the greatest risk of Wilms tumor are WAGR syndrome (Wilms tumour-aniridia-genitourinary malformation-retardation), Denys-Drash Syndrome, and Beckwith-Wiedemann Syndrome.
Familial Wilms tumor: Approximately 1.5 percent of children with Wilms tumor have a relative who was also affected with Wilms tumor.
Bilateral Wilms tumour: About 7 percent of patients with Wilms tumor have multiple tumors in one or both kidneys. Having multiple tumors does not indicate tumor spread.
Girls are at a slightly higher risk than boys of developing a Wilms' tumor.
The tumor is contained within one kidney and can be completely removed by surgery. The tissue layer surrounding the kidney (the renal capsule) is not broken and the cancer has not grown into blood vessels in or next to the kidney.
About 40% to 45% of all Wilms tumors are stage I.
The tumor has grown beyond the kidney, either into nearby fatty tissue or into blood vessels in or near the kidney, but can be completely removed by surgery without any apparent cancer left behind. Lymph nodes do not contain tumor.
About 20% of all Wilms tumors are stage II.
This stage refers to Wilms tumors that have spread to the lymph nodes or that may have ruptured or spread to localized tissues or structures. One or more of the following features may be present:
The cancer has spread to lymph nodes (bean-sized collections of immune cells) in the abdomen or pelvis but not to more distant lymph nodes, such as those inside the chest.
The cancer has invaded nearby vital structures so the surgeon could not completely remove it.
Deposits of tumor (tumor implants) are found along the lining of the abdominal space.
Cancer cells are found at the edge of the sample removed by surgery, indicating that some of the cancer still remains after surgery.
The cancer "spilled" into the abdominal space before or during surgery.
The tumor was removed in more than one piece – for example, the tumor was in the kidney and in the nearby adrenal gland, which was removed separately.
About 20% to 25% of all Wilms tumours are stage III.
Wilms Tumors are staged depending upon where the tumor has spread in the body and also what features they contain when looked at closely through a microscope.
The cancer has spread through the blood to organs away from the kidneys such as the lungs, liver, brain, or bone, or to lymph nodes far away from the kidneys. Tumors in other parts of the body are known as metastases.
About 10% of all Wilms tumours are stage IV.
Tumors are found in both kidneys at diagnosis. Also called bilateral Wilms tumors.
About 5% of all Wilms tumors are stage V.
Wilms (or ‘nephroblastoma’): Staging I - IV
Types of Wilms Tumors
There are 2 main types of Wilms’ tumor. The cells in each type look different under a microscope. Doctors call this the histology of the tumour. The 2 types are:
Wilms' tumor with favourable histology
Wilms' tumor with unfavourable histology
Unfavourable histology means that the cells look very large and not like normal kidney cells. The medical term for this is anaplasia. The cancer is less likely to be cured if there are lots of areas of anaplasia. But more than 9 out of 10 Wilms’ tumors (95%) have favourable histology. This means that there is no anaplasia and the chance of cure is high.
Several other very rare types of kidney cancers are found in children. Doctors used to group these as Wilms’ tumors with unfavourable histology but they are now grouped separately. They are treated in the same way as a Wilms' tumor but often more intensively. They are:
Clear cell sarcoma of the kidney (CCSK)
Malignant rhabdoid tumour of the kidney
Wilms (or ‘nephroblastoma’): Normal and diseased kidney cells
Recurrent or relapsed Wilms tumors are not staged.
Based on the biopsy result, and after examining the whole tumor under the microscope, Wilms’ tumors can be divided into a number of groups based on knowledge about how these different types of tumors are likely to behave.
The tumor will be classified as 'low risk' 'intermediate risk' or 'high risk' depending on how likely, based upon what they have seen under the microscope, the tumor is likely to respond to treatment and if it is likely to relapse.
Certain factors result in a 'high risk' Wilms’ tumor. High risk tumors require more intensive (stronger) chemotherapy and closer observation following treatment:
Anaplastic Wilms’ tumor
About 5-10% of Wilms’ tumors have an appearance called anaplasia, which means the cells look very disorganized under a microscope. This is sometimes identified at biopsy, but may only be found when the whole tumor is examined after surgery.
Blastemal Wilms’ tumour
This group of high-risk tumors cannot be identified by looking at the biopsy because they occur when a particular type of early kidney cell survives the pre-surgery chemotherapy. These cells are known as blastemal cells. Tumors where most of these cells survive chemotherapy are called blastemal tumours.
Loss of Heterozygosity (LOH)
Information researchers have learnt about changes in genes of children with Wilms tumor have found that specific gene mutations or abnormalities can potentially play a role in tumor growth and tumour control and may also provide information about prognosis and risk of relapse.
Children whose tumors show loss of heterozygosity (a situation where one chromosome has a normal copy of a gene and one chromosome has a mutant or deleted copy, at chromosomes 16q and 1p, for example) appear to have worse survival rates and are more likely to relapse.