Nephroblastoma (Wilms Tumor): Understanding Your Pathology Report

Section Editor: Trevor Flood MD FRCPC
July 18, 2026

Nephroblastoma, also called Wilms tumor, is a type of kidney cancer. It is the most common kidney cancer in children, and it is rare in adults. The tumor develops from early kidney cells called nephrogenic blastema, the cells that normally build the kidney before birth. Under the microscope, the tumor looks like a kidney that is still forming, which is where its name comes from.

The outlook for children with nephroblastoma is very good. With modern treatment, the great majority are cured. Because this is a childhood cancer, this article is written for parents and caregivers as well as for older patients. It will help you understand the findings in your child’s pathology report, what each term means, and why it matters for their care. If you are an adult reading your own report, the same information applies to you.

How common is nephroblastoma?

Nephroblastoma, the most common kidney cancer in children, is most often diagnosed between the ages of 3 and 4. It is slightly more common in girls than in boys and affects about 1 in 10,000 children in Europe and North America. It is more common in children of African ancestry and less common in children of East Asian ancestry. It rarely occurs in adults, and when it does, it is treated by teams with specific expertise.

Where does nephroblastoma occur?

Nephroblastoma develops in the kidney. In most children, only one kidney is affected. In about 5 to 10 out of every 100 children, both kidneys are involved, which is called bilateral disease and is staged separately (see the staging section below). Very rarely, the tumor develops outside the kidney.

What are the symptoms of nephroblastoma?

The most common sign of nephroblastoma, a kidney cancer of childhood, is a lump or swelling in the abdomen, often first noticed by a parent while bathing or dressing the child. The child usually feels well, which is one reason the swelling can be a surprise. About 1 in 4 children also develop other symptoms, which may include:

Abdominal pain.
Blood in the urine, which can make the urine look pink, red, or brown.
High blood pressure, which the tumor can cause by releasing a hormone that regulates blood pressure.
A low red blood cell count (anemia), which can cause tiredness and paleness.

What causes nephroblastoma?

Nephroblastoma is thought to develop from small clusters of leftover early kidney cells called nephrogenic rests. These are groups of cells that should have matured into normal kidney before birth but instead remained behind. Nephrogenic rests are found in about 40 out of every 100 children with a tumor in one kidney, and in more than 90 out of every 100 children with tumors in both kidneys. Most nephrogenic rests never become cancer, but some do, and when many are present, the condition is called nephroblastomatosis. Certain patterns of rests, particularly in infants, carry a higher risk of a tumor developing in the other kidney, which is one reason the remaining kidney is watched closely after treatment.

Several genes are involved in the development of nephroblastoma, including WT1, WT2, CTNNB1, WTX, TP53, and MYCN. Changes in these genes cause early kidney cells to continue growing rather than mature. Most of these changes happen in the tumor itself, but in some children they are part of an inherited condition, described in the next section.

Are some children more likely to develop nephroblastoma?

Yes. In about 10 to 15 out of every 100 children, nephroblastoma is linked to an inherited genetic condition, or syndrome. These conditions matter because they change how closely a child is monitored, they may affect the other kidney, and they have implications for the family. A child with one of these conditions is often screened with regular kidney ultrasounds from an early age, so that any tumor is found early.

Conditions with a high risk (more than 20 out of 100 develop a tumor):

WAGR syndrome — A combination of Wilms tumor, absence of the colored part of the eye (aniridia), genital and urinary abnormalities, and developmental delay, caused by loss of a region of chromosome 11 that includes the WT1 gene.
Denys-Drash syndrome — Caused by a change in the WT1 gene, combining Wilms tumor with kidney disease and genital abnormalities.

Conditions with a moderate risk (5 to 20 out of 100):

Beckwith-Wiedemann syndrome — An overgrowth condition involving a region of chromosome 11 (11p15), causing a larger birth size, a large tongue, and an increased risk of several childhood tumors.
Simpson-Golabi-Behmel syndrome — Another overgrowth condition.
Frasier syndrome — Also caused by a change in the WT1 gene.

Conditions with a lower risk (less than 5 out of 100):

Isolated hemihypertrophy, where one side of the body grows larger than the other.
Bloom syndrome, DICER1 syndrome, and Li-Fraumeni syndrome.

In addition, about 1 to 2 out of every 100 cases occur in families with a history of nephroblastoma. Because of these links, a referral to a genetics specialist is a routine part of care, especially for a child who is very young, has tumors in both kidneys, or has any of the physical features above.

How is the diagnosis made?

The diagnosis of nephroblastoma is made when a pathologist examines the tumor under a microscope, usually after it has been removed by surgery. The tumor is first seen as a mass in the kidney on an ultrasound, CT scan, or MRI, and imaging also checks the other kidney, the large veins, and the lungs, which are the most common site of spread. Under the microscope, the pathologist looks for the three building blocks of a developing kidney: blastemal cells, epithelial cells, and stromal tissue (all three are described in the next section). Finding at least two of these usually confirms the diagnosis. When the appearance is not clear-cut, a test called immunohistochemistry, which uses special stains to detect proteins inside cells, or molecular testing may be added to separate nephroblastoma from other childhood kidney tumors.

A point that often surprises parents is that a needle biopsy before treatment is usually avoided in nephroblastoma. The reason is that puncturing the tumor can allow cells to spill, which would raise the tumor’s stage and lead to more intensive treatment. Instead, the diagnosis is typically made from imaging and the entire tumor after it is removed. A biopsy is considered only in unusual situations, such as a child older than 10 years, a very large tumor, or features on imaging that raise the possibility of a different diagnosis. Whether surgery or chemotherapy comes first depends on the treatment approach used, which is explained in the “What happens after the diagnosis?” section.

What does nephroblastoma look like under the microscope?

Nephroblastoma is classically composed of three components: the same tissues that form a normal kidney before birth. A single tumor may contain all three, or only one or two; the mix is noted in the report because it can influence risk grouping.

Blastemal cells — Small, immature cells that resemble the earliest kidney tissue. When these cells make up most of a tumor that has already been treated with chemotherapy, the tumor is considered higher risk.
Epithelial cells — Cells that form tubes and filtering-unit shapes, resembling the parts of a kidney that carry urine.
Stromal tissue — Supporting connective tissue, which may contain muscle, fat, or cartilage.

The single most important microscopic feature is anaplasia, found in about 7 to 10 out of every 100 nephroblastomas. Anaplasia means that the tumor cells are much larger and more abnormal than usual, with strikingly enlarged nuclei and abnormal mitotic figures. It is described as focal (present in only a limited area) or diffuse (spread across the tumor). Diffuse anaplasia is the main feature that marks a tumor as harder to treat, and the pathologist searches the whole tumor carefully for it. Its presence or absence is the basis of the histology classification described below.

Immunohistochemistry

When immunohistochemistry is used, the three components each show a characteristic pattern. Blastemal cells are usually positive for WT1, PAX8, and vimentin; epithelial cells are positive for cytokeratins, EMA, and CD56; and stromal cells are positive for vimentin and sometimes CD34. The WT1 protein is present in about 90 out of every 100 nephroblastomas and is especially helpful. These stains confirm the diagnosis and distinguish nephroblastoma from other childhood kidney tumors; they do not guide treatment, which is why they belong here in the diagnosis section rather than in the biomarker section.

How doctors classify nephroblastoma

Classification means grouping tumors by how they look under the microscope, and it predicts how the tumor will behave and how much treatment is needed. Two systems are used, and which one applies to your child depends on where they are treated and, importantly, on whether chemotherapy was given before surgery. This is the single biggest reason two pathology reports for nephroblastoma can look different, so it is worth understanding.

COG classification (used mainly in the United States and Canada)

In the Children’s Oncology Group (COG) approach, surgery is usually performed first, so the pathologist examines an untreated tumor. The classification is simple:

Favorable histology — No anaplasia. These tumors respond very well to treatment and account for the vast majority of cases.
Anaplastic histology — Diffuse anaplasia is present. These tumors need stronger treatment.

SIOP classification (used in Europe and many other countries)

In the International Society of Paediatric Oncology (SIOP) approach, chemotherapy is given before surgery, so the pathologist examines a tumor that has already partly responded to treatment. The classification reflects how much of the tumor was destroyed and what remains:

Low risk — Tumors almost completely destroyed by chemotherapy, or certain cystic tumors.
Intermediate risk — Tumors with a mix of components, partially destroyed tumors, or focal anaplasia.
High risk — Tumors made mostly of surviving blastemal cells after chemotherapy, or tumors with diffuse anaplasia.

If your child’s tumor was treated with chemotherapy first, the report may describe how much of the tumor is dead (non-viable) versus alive (viable), because a tumor that responded well is a favorable sign. A tumor made mostly of surviving blastemal cells after chemotherapy is one that resisted treatment, which is why it is placed in the high-risk group.

How doctors stage nephroblastoma

Staging describes how far the tumor has spread at the time of diagnosis, and it guides treatment. Nephroblastoma is not staged with the TNM system used for adult cancers. Instead, both the COG and SIOP approaches use a five-stage system that is broadly similar, based on the surgeon’s findings and the pathologist’s examination.

Stage I — The tumor is limited to the kidney and was completely removed.
Stage II — The tumor grew into tissue just outside the kidney, such as the surrounding fat or the renal sinus, but was still completely removed.
Stage III — Tumor remains in the abdomen after surgery. This includes tumor at the cut edge of the removed tissue, tumor in nearby lymph nodes, or tumor that ruptured or spilled before or during surgery.
Stage IV — The tumor has spread to distant organs, most often the lungs, or to lymph nodes outside the abdomen.
Stage V — Tumors are present in both kidneys at diagnosis. Each kidney is also given its own stage.

Two findings in this list deserve emphasis because they change the stage on their own. Tumor rupture or spill, whether it happens on its own before surgery or during the operation, places the tumor at stage III and leads to more intensive treatment, which is the main reason surgeons handle these tumors so carefully and avoid biopsy. Cancer in a lymph node also raises the stage and is one of the most important findings the pathologist reports, so lymph nodes are routinely sampled during surgery for nephroblastoma, unlike the surgery for most adult kidney cancers.

Biomarker and molecular testing

Biomarkers are features of a tumor, usually changes in genes or chromosomes, that provide information beyond the diagnosis itself. Nephroblastoma is unusual among the kidney cancers on this site because specific genetic findings in the tumor are used directly to adjust treatment. These are tested on the tumor tissue and reported separately from the microscope findings.

Loss of heterozygosity at 1p and 16q

Loss of heterozygosity means that a piece of one of the two copies of a chromosome has been lost from the tumor cells. In favorable-histology nephroblastoma, the loss of pieces of both chromosome 1p and chromosome 16q together identifies tumors more likely to come back. In the COG approach, this combined finding is used to intensify chemotherapy because studies have shown that adding stronger treatment improves outcomes for exactly this group. A report may state that both are lost, that one is lost, or that neither is lost; the combined loss of both is the finding that changes treatment.

Gain of 1q and other chromosome changes

An extra copy of part of chromosome 1q (called 1q gain) is associated with a higher risk of tumor recurrence and is an area of active study to guide treatment. Changes in a region of chromosome 11 called 11p15, which is also implicated in Beckwith-Wiedemann syndrome, carry prognostic information. These findings are reported when comprehensive testing is performed. They are used alongside stage, histology, and age to place a child in a risk group.

Inherited (germline) genetic testing

Separate from tests on the tumor, testing of a blood or saliva sample may be recommended to look for an inherited condition, particularly for a child who is very young, has tumors in both kidneys, or has physical features suggesting one of the syndromes described earlier. A change found in this testing has implications for the child’s other kidney, for long-term monitoring, and for the family, and it is arranged through a genetics clinic. You can learn more about the tests described here in our Biomarkers and Genetic Testing section.

What is the prognosis?

Prognosis means the expected course of a disease. The outlook for children with nephroblastoma is generally very good. With modern treatment, about 9 out of 10 children are cured, and even many children with higher-risk disease do well. When the tumor does come back, it usually happens within the first two years after diagnosis, which is why follow-up is most intensive during that time. Prognosis depends on a combination of findings rather than any single one.

Features linked with a better outcome:

Stage I or II disease, limited to the kidney or nearby tissue and completely removed.
Favorable histology (no anaplasia).
Younger age, particularly under 2 years, with a small, low-stage tumor.
Lung deposits that shrink quickly after the first rounds of chemotherapy.
No high-risk chromosome changes in the tumor.

Features linked with a worse outcome:

Stage III disease with tumor in lymph nodes, rupture, or tumor that could not be fully removed.
Stage IV disease with spread to distant organs.
Stage V disease affecting both kidneys, which makes surgery more complex.
Diffuse anaplasia.
Older age, especially older than 10 years.
Lung deposits that do not shrink with initial chemotherapy.
Combined loss of heterozygosity at 1p and 16q, or gain of 1q.

A small, very-low-risk group, generally children under 2 years with a small stage I favorable-histology tumor, may be treated with surgery alone and no chemotherapy, reflecting how well these tumors do. Your child’s oncology team combines all of these findings to decide how much treatment is needed, aiming to cure the cancer while sparing a young child unnecessary treatment.

What happens after the diagnosis?

Nephroblastoma is treated by a specialized children’s cancer team, usually including a pediatric oncologist, a pediatric surgeon or urologist, a radiation oncologist, a pathologist, a radiologist, and a genetic counselor. Treatment combines surgery, chemotherapy, and sometimes radiation, and the order depends on the approach used.

The two treatment approaches — In the COG approach, used mainly in the United States and Canada, surgery to remove the kidney is usually done first, and the pathology findings then guide chemotherapy and, for some children, radiation. In the SIOP approach, used in much of the rest of the world, chemotherapy is given first to shrink the tumor, followed by surgery and further treatment. Both achieve excellent results. Which one your child receives depends on where they are treated, and it explains why the pathology report is organized around either an untreated tumor (COG) or a treated one (SIOP).
Surgery — Most children have the affected kidney removed. When both kidneys are involved, surgeons work to remove the tumor while preserving as much working kidney as possible, often after chemotherapy has shrunk the tumors. Lymph nodes are sampled during surgery because node involvement changes the stage and the treatment.
Chemotherapy — Nearly all children receive chemotherapy, most commonly a combination that includes vincristine and actinomycin D, with doxorubicin added for higher-risk tumors and additional drugs for the highest-risk groups. The intensity is matched to the stage, the histology, the age, and the chromosome findings.
Radiation therapy — Radiation is given to some children, typically those with higher-stage disease, tumor left behind in the abdomen, or spread to the lungs.
Follow-up and long-term care — After treatment, children are followed with regular imaging of the chest and abdomen, most intensively in the first two years when recurrence is most likely. Because these children are treated young and are expected to be cured, long-term follow-up also watches for late effects of treatment on the heart, kidneys, growth, and fertility, and for the small risk of a second cancer later in life. If an inherited condition was found, the other kidney is monitored and family members may be offered testing.

Questions to ask your doctor

What stage is the tumor?
Was the histology favorable, or was anaplasia found, and was it focal or diffuse?
Which treatment approach is being used, COG or SIOP, and does that affect how I should read the report?
Was chemotherapy given before surgery, and if so, how well did the tumor respond?
Did the tumor rupture or spill before or during surgery?
Were lymph nodes removed, and did any contain tumor?
Were the surgical margins clear, or was tumor found at the edge?
Was the tumor tested for chromosome changes such as loss at 1p and 16q or gain of 1q?
Will my child need chemotherapy, radiation, or both, and for how long?
Is genetic testing recommended for my child or our family?
Is the other kidney being monitored?
What follow-up will be needed, and what late effects of treatment should we watch for?