Wilms tumor is the most common kidney cancer in children, with roughly 650 new cases diagnosed in the United States each year. It typically appears as a large mass growing in one or both kidneys, most often in toddlers and young children. Ninety percent of cases are caught before age six, and the median age at diagnosis is 3.5 years.
The good news: Wilms tumor is one of the great success stories in pediatric cancer treatment. Over 90% of children with the most common form survive long term. But the road from diagnosis through treatment and beyond involves real decisions and real consequences worth understanding.
How Wilms Tumor Is Discovered
Most parents first notice something is wrong when they feel or see a firm, swollen area in their child’s belly. A doctor’s exam typically reveals a palpable abdominal mass, sometimes large enough to be visible. In many cases, the mass is found incidentally during a routine checkup or while a parent is bathing or dressing their child.
Beyond the abdominal swelling, 5% to 30% of children will also have high blood pressure, blood in the urine, or fever. These symptoms can mimic a urinary tract infection, which sometimes delays the correct diagnosis. In rare cases where the tumor has bled internally, a child may appear pale, have low blood pressure, or develop a sudden fever. A small number of children with advanced disease show up with breathing difficulties from tumor spread to the lungs.
What Causes It
Wilms tumor arises from immature kidney cells that failed to develop normally during fetal growth. The genetics behind it are complex and not fully mapped. Known gene mutations account for only about one-third of cases, meaning most tumors develop through pathways scientists haven’t yet pinpointed.
The best-understood gene involved is WT1, a tumor suppressor gene on chromosome 11 that is mutated in roughly 20% of cases that occur without any associated syndrome. A second gene called WTX is altered in about 18% of tumors, and a third gene, CTNNB1, which encodes a protein involved in cell signaling and adhesion, is mutated in approximately 15%. These mutations can overlap within the same tumor, but together they still leave two-thirds of Wilms tumors genetically unexplained.
A small percentage of children develop Wilms tumor as part of a broader genetic syndrome. The most well-known is WAGR syndrome, which combines Wilms tumor risk with absence of the iris in the eye, genital and urinary abnormalities, and intellectual disability. Beckwith-Wiedemann syndrome, characterized by overgrowth and enlarged organs, also raises the risk. Denys-Drash syndrome, which affects kidney function and genital development, carries a particularly high association. Children with these syndromes are monitored closely with regular kidney ultrasounds starting in infancy.
How It Is Diagnosed
When a doctor suspects Wilms tumor based on a physical exam, imaging comes next. Ultrasound is usually the first step, giving a quick look at whether a mass is present in the kidney. CT scans or MRI then provide a detailed picture of the tumor’s size, whether it has spread to nearby lymph nodes, and whether the other kidney is involved.
MRI is increasingly valuable because it can distinguish Wilms tumor from neuroblastoma, a different childhood cancer that arises near the kidneys but from nerve tissue rather than kidney tissue. Advanced MRI techniques that measure how water molecules move through tissue can differentiate between these tumor types based on their internal structure, which helps doctors plan the right treatment from the start.
Staging and Treatment Approach
Two major treatment philosophies exist worldwide, and which one your child encounters depends largely on geography. In North America, the standard approach (developed through the Children’s Oncology Group) is to surgically remove the affected kidney first. This gives doctors a precise picture of the tumor’s stage and cell type before deciding on further treatment.
In much of Europe and other parts of the world, the approach developed by the International Society of Paediatric Oncology (SIOP) starts with several weeks of chemotherapy before surgery. The goal is to shrink the tumor, making removal safer and reducing the chance of the tumor rupturing during the operation. Both approaches produce excellent survival rates.
Staging runs from I through V. Stage I means the tumor was completely contained within the kidney and removed cleanly. Stage II means it grew beyond the kidney but was still fully removed. Stage III indicates some tumor was left behind or had spread to nearby lymph nodes. Stage IV means it spread to distant organs like the lungs. Stage V means both kidneys are affected, which occurs in about 5% to 7% of cases.
What Treatment Looks Like
Surgery to remove the affected kidney (nephrectomy) is the cornerstone of treatment. Children can live completely normal lives with one healthy kidney, so losing one is not as alarming as it might initially sound. In bilateral cases where both kidneys are involved, surgeons try to preserve as much healthy kidney tissue as possible through partial removal.
Nearly all children also receive chemotherapy. The standard combination for lower-stage, favorable tumors uses two drugs that target rapidly dividing cells. Children with higher-stage disease or more aggressive tumor types receive additional agents. Treatment duration varies by stage but generally spans several months. Radiation therapy is added for stage III and IV disease, or when the tumor’s cell type is more aggressive.
Favorable vs. Unfavorable Histology
The single most important factor in predicting outcome is what the tumor cells look like under a microscope. Most Wilms tumors have “favorable histology,” meaning the cells, while cancerous, don’t show extreme abnormalities. Children with favorable histology have survival rates above 90%, even at higher stages.
A smaller group of tumors show a pattern called anaplasia, where the cells are highly distorted and divide more aggressively. This is classified as “unfavorable histology,” and the four-year overall survival drops to about 74%. These tumors require more intensive chemotherapy and radiation. Knowing which type your child has is one of the key reasons the initial surgery and tissue analysis matter so much.
Long-Term Health After Treatment
Surviving Wilms tumor is the expected outcome for most children, which makes the long-term side effects of treatment especially important to understand. Survivors need monitoring that continues well into adulthood.
Heart Health
One of the chemotherapy agents used for higher-risk Wilms tumors can damage heart muscle cells. The risk increases with higher total doses. In a large study of children treated across several national trials, 4.4% of those who received this drug at initial diagnosis developed heart failure within 20 years. For those who needed the drug again at relapse, that figure rose to 17.4%. Even among children who appeared healthy, about one in four showed subtle signs of reduced heart function on testing. Girls face a higher risk than boys. Ongoing cardiac monitoring with echocardiograms is standard for these survivors.
Kidney Function
Losing one kidney puts more demand on the remaining one, and most children compensate well. Among those with unilateral Wilms tumor and no underlying genetic conditions, the chance of developing kidney failure within 20 years is low, around 0.6%. For bilateral disease, that risk climbs to 12%. Children with underlying genetic syndromes face significantly higher rates: 74% for Denys-Drash syndrome and 36% for the WAGR-associated form within 20 years. Radiation to the area of the remaining kidney and incomplete compensatory growth of that kidney are additional risk factors. Chronic kidney insufficiency of varying severity has been reported in 19% to 73% of survivors depending on the treatment they received.
Because of these risks, long-term survivors benefit from regular blood pressure checks, urine tests, and kidney function monitoring throughout their lives. Protecting the remaining kidney from unnecessary injury, including avoiding contact sports that carry a risk of direct kidney trauma, is a practical consideration many families discuss with their child’s care team.