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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 callederythropoietin, 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.
Mesoblastic nephroma: 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.
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.
What are the risk factors for Wilms tumor?
A risk factor is anything that affects the chance of having a disease such as cancer. Different cancers have different risk factors. For example, smoking is a risk factor for several types of cancer in adults.
Lifestyle-related risk factors such as body weight, physical activity, diet, and tobacco use play a major role in many adult cancers. But these factors usually take many years to influence cancer risk, and they are not thought to have much of an effect on the risk of childhood cancers, including Wilms tumors.
So far research has not found any strong links between Wilms tumor and environmental factors, either during a mother's pregnancy or after a child's birth.
Most Wilms tumors have no clear cause.
Age: Wilms tumors are most common in young children, with the average age being about 3 to 4 years. They are less common in older children, and are rare in adults.
Race: In the United States, the risk of Wilms tumor is slightly higher in African-American children than in white children and is lowest among Asian-American children. The reason for this is not known.
Gender: The risk of Wilms tumor is slightly higher in girls than in boys.
Family history of Wilms tumor: About 1% to 2% of children with Wilms tumors have one or more relatives with the same cancer. Scientists think that these children inherit chromosomes with an abnormal or missing gene from a parent that increases their risk of developing Wilms tumor. Surprisingly, the relative with Wilms tumor is not usually a parent.
Children with a family history of Wilms tumors are slightly more likely to have tumors in both kidneys. Still, in most children only one kidney is affected.
Certain inherited syndromes/birth defects: There is a strong link between Wilms tumors and certain kinds of birth defects. About 1 child in 10 with Wilms tumor also has birth defects. Most birth defects linked to Wilms tumors occur in syndromes. A syndrome is a group of symptoms, signs, malformations, or other abnormalities that occur together in the same person. Syndromes linked to Wilms tumor include:
WAGR syndrome: WAGR stands for the first letters of the physical and mental problems linked with this syndrome (although not all children have all of them):
Aniridia (complete or partial lack of the iris [colored area] of the eyes)
Genitourinary tract abnormalities (defects of the kidneys, urinary tract, penis, scrotum, clitoris, testicles, or ovaries)
Children with this syndrome have about a 30% to 50% chance of having a Wilms tumor. The cells in children with WAGR syndrome are missing part of chromosome 11, where the WT1 gene is normally found.
Beckwith-Wiedemann syndrome :Children with this syndrome tend to be big for their age. They also have larger than normal internal organs and often have an enlarged tongue. They may have an oversized arm and/or leg on one side of the body (hemihypertrophy), as well as other medical problems. They have about a 5% to 10% risk of having Wilms tumors. This syndrome is also caused by a defect in chromosome 11.
Denys-Drash syndrome: This rare syndrome has been linked to changes (mutations) in the WT1 gene. In this syndrome the kidneys become diseased and stop working when the child is very young. Wilms tumors usually develop in the diseased kidneys. The reproductive organs do not develop normally, which in boys may cause them to be mistaken for girls. Because the risk of Wilms tumors is very high, doctors often advise removing the kidneys soon after this syndrome is diagnosed.
Other syndromes: Less often, Wilms tumor has been linked to other syndromes, including:
Certain birth defects
Wilms tumor is also more common in children with certain birth defects (without known syndromes):
Aniridia (complete or partial lack of the iris [colored area] of the eyes)
Hemihypertrophy (an oversized arm and/or leg on one side of the body)
Cryptorchidism (failure of the testicles to descend into the scrotum) in boys
Hypospadias (defect in boys where the urinary opening is on the underside of the penis)
Do we know what causes Wilms tumor?
Although there is a clear link between Wilms tumors and certain birth defect syndromes and genetic changes, most children with this type of cancer do not have any known birth defects or inherited gene changes.
Researchers do not yet know exactly why some children have Wilms tumors, but they have made great progress in understanding how normal kidneys develop, as well as how this process can go wrong, leading to a Wilms tumor.
The kidneys develop very early as a fetus grows in the womb. Changes (mutations) in certain genes in cells may lead to problems as the kidneys develop. Some of the cells that are supposed to develop into mature kidney cells stay as early kidney cells instead. Clusters of these early kidney cells sometimes remain after the baby is born. Usually, these cells mature by the time the child is 3 to 4 years old. If this does not happen, the cells may somehow begin to grow out of control. The result is a mass of primitive cells that may develop into a Wilms tumor.
Normal human cells grow and function based mainly on the information contained in each cell's chromosomes – long molecules of DNA in each cell. DNA is the chemical in each of our cells that makes up our genes – the instructions for how our cells function.
Some genes control when our cells grow, divide into new cells, and die. Certain genes that help cells grow and divide are called oncogenes. Others that slow down cell division or cause cells to die at the right time are called tumor suppressor genes. Cancers can be caused by DNA changes that turn on oncogenes or turn off tumor suppressor genes. These gene changes can be inherited from a parent (as is sometimes the case with childhood cancers) or may happen during a person's lifetime as cells in the body divide to form 2 new cells.
Changes in certain genes increase the chance that some kidney cells will remain in an early form and turn into a Wilms tumor.
For example, a small number of Wilms tumors are caused by inherited changes (mutations) in or loss of the WT1 orWT2 genes, which are tumor suppressor genes found on chromosome 11. Changes in these genes lead to overgrowth of certain tissues. That is why other abnormalities, like some of those described in the section, are sometimes found together with Wilms tumors.
But most Wilms tumors do not seem to be caused by inherited gene mutations. Instead, they seem to be the result of gene changes that occur early in a child's life, perhaps even before birth.
In a small number of Wilms tumors there is a change in a tumor suppressor gene known as WTX, which is found on the X chromosome. Another gene that is sometimes altered in Wilms tumor cells is known as CTNNB1. It's not clear exactly what causes these genes to be altered.
Because the genes described above are not altered in all cases of Wilms tumors, there must be changes in other genes that have not yet been found. In many cases, more than one gene change is involved.
Researchers now understand some of the gene changes that may occur in Wilms tumors, but it's still not clear what might cause these changes. Some gene changes may be inherited, but most Wilms tumors are not the result of known inherited syndromes. Other gene changes may just be random events that sometimes happen inside a cell, without having an external cause. There are no known lifestyle-related or environmental causes of Wilms tumors, so it is important to remember that there is nothing these children or their parents could have done to prevent these cancers.
Can Wilms tumor be prevented?
Although the risk of many adult cancers can be reduced with certain lifestyle changes (such as staying at a healthy weight or quitting smoking), at this time there are no known ways to prevent most cancers in children.
The only known risk factors for Wilms tumors (age, race, gender, and certain inherited conditions) cannot be changed. There are no known lifestyle-related or environmental causes of Wilms tumors, so at this time there is no way to protect against most of these cancers. Experts think these cancers come from cells that were in the fetus but failed to develop into mature kidney cells. This doesn't seem to be caused by anything that a mother could avoid.
In some very rare cases, such as in children with Denys-Drash syndrome who are almost certain to develop Wilms tumors, doctors may recommend removing the kidneys at a very young age (with a donor kidney transplant later on) to prevent tumors from developing.
Can Wilms tumor be found early?
Wilms tumors are usually detected when they start to cause symptoms such as swelling in the abdomen, but by this point they have often grown quite large. They can be detected earlier in some children with tests such as an ultrasound of the abdomen (a test that uses sound waves and a computer to make images of internal organs. But because Wilms tumors are so rare, it is not practical to do an ultrasound exam as a screening test (a test to look for disease in people with no signs or symptoms) in all children who are not at increased risk. There are no blood tests or other tests that are useful in screening otherwise healthy children for Wilms tumors.
On the other hand, screening for Wilms tumor is very important for children who have syndromes or birth defects known to be linked to this disease. For these children, most doctors recommend physical exams by a specialist and ultrasound exams on a regular basis (for example, every 3 or 4 months until the age of 7) to find any kidney tumors when they are still small and have not yet spread to other organs.
Wilms tumor can also run in families, although this is rare. Talk to your doctor if you have relatives who have had Wilms tumor. If you do, the children in your family may need to have regular ultrasound exams.
How are Wilms tumors diagnosed?
Wilms tumors are usually found when a child is brought to a doctor because of symptoms he or she is having. But even if the doctor is fairly sure a child has a Wilms tumor because of the results of a physical exam or other tests, the diagnosis is only made for certain once a sample of the tumor is removed and looked at under a microscope.
Signs and symptoms of Wilms tumor : Wilms tumors can be hard to find early because they can often grow quite large without causing any symptoms. Children may look healthy and play normally. The first sign is usually swelling or a hard mass in the abdomen (belly), which parents may notice while bathing or dressing the child. It feels firm and is often large enough to be felt on both sides of the belly. It is usually not painful, but it may cause belly pain in some cases. Some children with Wilms tumor may also have fever, nausea, loss of appetite, shortness of breath, constipation, or blood in the urine.
Wilms tumors can also sometimes cause high blood pressure. This does not usually cause symptoms on its own, but in rare cases it can get high enough to cause problems such as bleeding inside the eye or even a change in consciousness.
Many of the signs and symptoms above are more likely to be caused by something other than a kidney tumor. Still, if your child has any of these symptoms, check with your child's doctor so that the cause can be found and treated, if needed.
Medical history and physical exam : If your child has signs or symptoms that suggest he or she may have a kidney tumor, the doctor will want to get a complete medical history to learn more about the symptoms and how long they have been there. The doctor may also ask if there's a family history of cancer or birth defects, especially in the genitals or urinary system.
The doctor will examine your child to look for possible signs of Wilms tumor or other health problems. The main focus will likely be on the abdomen and on any increase in blood pressure, which is another possible sign of this cancer. Lab tests on samples of blood and urine may also be done at this time (see “Lab tests” below).
Imaging tests : If the doctor thinks your child might have a kidney tumor, he or she will probably order one or more of the imaging tests below. These tests use sound waves, x-rays, magnetic fields, or radioactive substances to create pictures of the inside of the body. Imaging tests are done for a number of reasons, including:
To help find out if there is a tumor in the kidney(s), and if so, if it is likely to be a Wilms tumor
To learn how far the tumor may have spread, both within the kidney and to other parts of the body
To help guide surgery or radiation therapy
To look at the area after treatment to help determine if it has been effective
Ultrasound (sonogram) : Ultrasound uses sound waves to create images of internal organs. For this test, your child lies on a table while a small, microphone-like instrument called a transducer is placed on the skin (which is first lubricated with a gel). It is moved across the skin over the kidney so that it can be viewed from different angles.
The transducer gives off sound waves and picks up the echoes as they bounce off the tissues in the kidney. The echoes are converted by a computer into a black and white image on a screen. The echo patterns made by most kidney tumors look different from those of normal kidney tissue. Different echo patterns also can help doctors distinguish some types of cancerous and non-cancerous kidney tumors from one another. In addition, ultrasound is very useful when looking for tumor thrombus (tumor growing into the main veins coming out of the kidney). This helps in planning for surgery, if it is needed.
This is often the first imaging test done if the doctor suspects your child has a Wilms tumor because it is easy to have, it does not use radiation, and it gives the doctor a good view of the kidneys and the other organs in the abdomen. The test is not usually painful, but it might cause some discomfort if the transducer is pressed down hard on the abdomen.
Computed tomography (CT, CAT) scan : The CT scan is an x-ray test that produces detailed cross-sectional images of parts of your child's body, including organs such as the kidneys. Instead of taking one picture, like a regular x-ray, a CT scanner takes many pictures as it rotates around your child while he or she lies on a table. A computer then combines these pictures into images showing slices of the part of the body being studied.
Before the scan, your child may be asked to drink a contrast solution and/or get an intravenous (IV) injection of a contrast dye that better outlines abnormal areas in the body. Your child may need an IV line for the dye. The contrast may cause some flushing (a feeling of warmth, especially in the face). Some people are allergic and get hives. Rarely, more serious reactions like trouble breathing or low blood pressure can occur. Be sure to tell the doctor if your child has any allergies or has ever had a reaction to any contrast material used for x-rays.
CT scans take longer than regular x-rays. Your child will need to lie still on a table while they are being done. During the test, the table slides in and out of the scanner, a ring-shaped machine that completely surrounds the table. Some people feel a bit confined by the ring they have to lie in while the pictures are being taken. Younger children may be given medicine to help keep them calm or even asleep during the test to help make sure the pictures come out well. Many medical centers now use spiral CT (also known as helical CT), which completes the scan more quickly. It also gives more detailed pictures and lowers the dose of radiation received during the test.
A CT scan is one of the most useful tests to look for a mass inside the kidney. It is also helpful in checking whether a cancer has spread to organs and tissues beyond the kidney, such as the lungs.
Magnetic resonance imaging (MRI) scan : Like CT scans, MRI scans provide detailed images of soft tissues in the body. But MRI scans use radio waves and strong magnets instead of x-rays (and therefore don't expose your child to radiation). The energy from the radio waves is absorbed and then released in a pattern formed by the type of body tissue and by certain diseases. A computer translates the pattern into a very detailed image of parts of the body.
A contrast material called gadolinium may be injected into a vein before the scan to better see details. The contrast material usually does not cause allergic reactions, but it can cause other problems in children with kidney disease, so doctors are careful when they use it.
MRI scans take longer than CT scans – often up to an hour. Your child may have to lie inside a narrow tube, which is confining and can be distressing. Newer, more open MRI machines may help with this, but the test still requires staying still for long periods of time. The MRI machine also makes loud buzzing and clicking noises that your child may find disturbing. Younger children may be given medicine to help keep them calm or even asleep during the test.
MRI scans show more detailed images than CT and ultrasound. They may be done if there's a chance that the cancer is in a major vein (the inferior vena cava) in the abdomen. They may also be used to look for possible spread of cancer to the brain or spinal cord if doctors are concerned the cancer may have spread there.
Chest x-ray: Chest x-rays may be done to look for any spread of Wilms tumor to the lungs, as well as to have a baseline view of the lungs to compare with other x-rays that might be done in the future. If a CT scan of the chest is done, this test is not needed.
Bone scan : Bone scans can help find cancer that has spread to bones. Doctors don't usually order this test unless they think your child has a type of Wilms tumor that is likely to spread.
For this test, a small amount of low-level radioactive material is injected into a vein (intravenously, or IV). The substance settles in areas of damaged bone throughout the entire skeleton over the course of a couple of hours. Your child then lies on a table for about 30 minutes while a special camera detects the radioactivity and creates a picture of the skeleton. Younger children may be given medicine to help keep them calm or even asleep during the test.
Areas of active bone changes appear as "hot spots" on the skeleton – that is, they attract the radioactivity. These areas may suggest the presence of cancer, but other bone diseases can also cause the same pattern. To distinguish between these conditions, other imaging tests such as plain x-rays or MRI scans, or even a bone biopsy might be needed.
Lab tests: Lab tests may be done to check urine and blood samples if your child’s doctor suspects a kidney problem. They may also be done after a diagnosis of Wilms tumor has been made.
A urine sample may be tested (urinalysis) to look for blood and other substances in the urine to see if there are problems with the kidneys. The urine may also be tested for substances called catecholamines. This is done to make sure your child doesn't have another kind of tumor called neuroblastoma. (Neuroblastomas often start in the adrenal gland, which lies just above the kidney.)
Blood tests are not used to find Wilms tumors, but they may be done to check a child's general health (especially before surgery) and to look for side effects during treatment such as chemotherapy. These may include tests to count the number of white blood cells, red blood cells, and blood platelets, and tests to measure certain chemicals and salts in the blood that give clues about how well the liver and kidneys are working.
Kidney biopsy/surgery : Most of the time, imaging tests can give doctors enough information to decide if a child probably has a Wilms tumor, and therefore surgery should be done. But the actual diagnosis of Wilms tumor is made when a sample of the tumor is removed and looked at under a microscope. The cells in Wilms tumors have a characteristic appearance when looked at this way. This is also when doctors determine the histology of the Wilms tumor (favorable or unfavorable).
In most cases, the sample is obtained during surgery to treat the tumor. Sometimes if the doctors are less certain about the diagnosis or if they are not sure the tumor can be removed completely, a sample of the tumor may be obtained first during a biopsy as a separate procedure. The biopsy may be done either surgically or using a long, hollow needle that is inserted through the skin and into the tumor.
How is Wilms tumor staged?
Staging is the process of finding out how far a cancer has spread. Your child's treatment and prognosis (outlook) depend, to a large extent, on the cancer's stage. Staging is based on the results of the physical exam and imaging tests (ultrasound, CT scans, etc.), as well as on the results of surgery to remove the tumor, if it has been done.
Children’s Oncology Group (COG) staging system
A staging system is a standard way for the cancer care team to sum up their findings of how extensive the tumor is. In the United States, the Children’s Oncology Group staging system is used most often to describe the extent of spread of Wilms tumors. This system describes Wilms tumor stages using Roman numerals I through V (1 through 5).
Stage I: The tumor was contained within one kidney and was completely removed by surgery. The tissue layer surrounding the kidney (the renal capsule) was not broken during surgery. The cancer had not grown into blood vessels in or next to the kidney. The tumor was not biopsied before surgery to remove it.
About 40% to 45% of all Wilms tumors are stage I.
Stage II: The tumor has grown beyond the kidney, either into nearby fatty tissue or into blood vessels in or near the kidney, but it was completely removed by surgery without any apparent cancer left behind. Lymph nodes do not contain tumor. The tumor was not biopsied before surgery.
About 20% of all Wilms tumors are stage II.
Stage III: This stage refers to Wilms tumors that may not have been completely removed. The cancer remaining after surgery is limited to the abdomen (belly). 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.
A biopsy of the tumor was done before it was removed with surgery.
About 20% to 25% of all Wilms tumors are stage III.
Stage IV: 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.
About 10% of all Wilms tumors are stage IV.
Stage V : Tumors are found in both kidneys at diagnosis.
About 5% of all Wilms tumors are stage V.
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