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Wilms tumors and the kidneys

To understand Wilms tumors, it helps to know about the kidneys and how they work.

The kidneys are 2 bean-shaped organs attached to the back wall of the abdomen (belly), just to the left and right of the backbone. The job of the kidneys is to:

• Filter the blood to remove excess water, salt, and waste products, which leave the body as urine
• Help control blood pressure
• Help make sure the body has enough red blood cells

On top of each kidney is a small gland called an adrenal gland. The adrenal glands make hormones that have many functions, including helping the body burn fat and protein and respond to stress.

Each kidney and adrenal gland is surrounded by fat and a thin, fibrous capsule (known as Gerota’s fascia). They are protected by the lower rib cage.

Our kidneys are important, but we actually need less than one complete kidney to do all of its basic functions. Many people in the United States live normal, healthy lives with just one kidney.

How Wilms tumors form

The kidneys develop very early as a fetus grows in the womb. Certain cells are programmed to turn into mature kidney cells. But sometimes, some of these early kidney cells don’t mature like they are supposed to. Instead, they stay in the body in the form of early (not mature) kidney cells. They might stay in the body even after the baby is born.

Usually, these cells mature by the time the child is 3 to 4 years old. But if this doesn’t happen, the cells might somehow begin to grow out of control, which could result in a Wilms tumor.

How Wilms tumors are grouped

Wilms tumors can be grouped in different ways:

Unilateral versus bilateral Wilms tumors

Wilms tumors can be either unilateral or bilateral.

• Unilateral: Most Wilms tumors are unilateral, which means they affect only one kidney. Most often there is only one tumor. But a small number of children with Wilms tumors have more than one tumor in the same kidney.
• Bilateral: About 5% to 10% of children with Wilms tumors have bilateral disease (tumors in both kidneys). These tumors often appear in children at an earlier age than unilateral tumors.

Rarely, a Wilms tumor develops in one kidney first and then in the other.

Wilms tumor histology

Wilms tumors are grouped into 2 major types, based on how they look under a microscope (their histology).

• Favorable histology: The cancer cells in these tumors don’t look quite normal, but there is no anaplasia (see next paragraph). About 9 of 10 Wilms tumors have a favorable histology. The chance of curing children with these tumors is very good.

• Anaplastic histology: In these tumors, 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.

In general, tumors in which the anaplasia is spread throughout the tumor (known as diffuse anaplasia) are harder to treat than tumors in which the anaplasia is limited just to certain parts of the tumor (known as focal anaplasia).

Other types of kidney cancers in children

Rarely, children can develop other types of kidney tumors.

Mesoblastic nephroma

These tumors usually appear in the first few months of life. Children with these tumors are usually cured with surgery, but sometimes chemotherapy or targeted therapy is given as well. These tumors sometimes come back soon after treatment, so children who’ve had them need to be watched closely for the first year afterward.

Clear cell sarcoma of kidney (CCSK)

These tumors are much more likely to spread to other parts of the body than Wilms tumors. They are also harder to cure. Because these tumors are rare, treatment is often given as part of a clinical trial. It's usually a lot like the intensive treatment used for Wilms tumors with anaplastic histology (see Treatment of Wilms Tumors by Type and Stage).

Malignant 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. 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. It 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 cancers in children. It’s rare in young children. But in older teens, it’s actually more common than Wilms tumor.

Surgery to remove the kidney (or just the tumor) is the main treatment for these cancers, if it can be done. The outlook depends largely on the extent (stage) of the cancer at the time it’s found, whether it can be removed completely with surgery, and its subtype (based on how the cancer cells look under a microscope).

If the cancer is too advanced to be removed by surgery, other types of treatment (such as targeted therapy and/or immunotherapy) may be needed.