In general terms, leukemia is a cancer of blood cells. Since all blood cells are derived from the bone marrow—the spongy space inside of bones—this is the anatomic origin of leukemia. But, leukemia is considered a "liquid" tumor in that it travels in the bloodstream (and therefore to all tissues) and is not based solely in one organ the way some other cancers are. Leukemia is classified in two different ways: (1) based on the type of bone marrow cell from which it originates, either lymphoid or myeloid, and (2) based on the stage of maturation of the tumor cell. Leukemia that arises from immature cells—so-called acute leukemia—is much more common in children than leukemia that arises from more mature cells—chronic leukemia. In acute leukemia, the leukemia cell is called a "blast" because it is very immature. This overview will focus on the two most common types of childhood leukemia, acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). Within these two broad groups there are many subtypes, some of which are described below.
Leukemia is the most common type of cancer in children - ALL accounts for approximately 75% of all childhood leukemia, and AML about 20%. While both of these major subtypes have similar presenting symptoms and initial diagnostic evaluations (and will be reviewed together in these respects), the treatments differ dramatically.
Like most childhood cancers, leukemia most commonly occurs randomly—or sporadically—in otherwise healthy children without known risk factors. There is a peak in the occurrence of ALL in children aged 2-4 years, but it is not known why this occurs. Certain subtypes of ALL also seem to affect different age groups; for example, T-cell ALL is more common in adolescents compared to younger children.
Certain underlying medical conditions can predispose to the development of leukemia. Children with certain rare inherited immunodeficiency syndromes are at increased risk for ALL. Exposure to specific types of chemotherapy or radiation (to treat cancer) increases a child's risk of developing AML. Children with Trisomy 21 (Down syndrome) have a greatly increased risk of developing both ALL and AML.
Patients with leukemia may have many different types of symptoms before they are diagnosed, but there are a few common clinical features that make doctors suspect that leukemia may be present. Because leukemia arises from the bone marrow where normal blood cells (white blood cells, red blood cells, and platelets) are made, one of the most common features associated with the diagnosis of acute leukemia is a decrease in these normal cells. This occurs when the leukemia blasts literally take up too much space in the bone marrow for the other normal cells to be produced. When normal white blood cells are low, children can develop unexplained fevers or infections; low red blood cells can cause children to develop signs of anemia such as sleepiness, fatigue, pale skin/eyes, headaches, or dizziness; and low platelets can cause bruising, nosebleeds, or gum bleeding. Frequently, leukemia cells (blasts) overflow from the marrow and can be seen by a hematologist or oncologist when the blood is examined under the microscope. Leukemia cells can also cause enlargement of liver and spleen, as well as lymph nodes in the neck, armpit, and groin.
In addition, because leukemia cells expand in the bone marrow space, they can exert pressure on the surrounding normal bone, causing pain. In small children this may manifest as a limp or refusal to walk or use an extremity.
If a doctor suspects that a child may have leukemia, the first step after a thorough history and physical exam, is typically to obtain a panel of blood tests, including blood counts (to determine if leukemia cells are present in the blood, and if the white cells, red cells, or platelets are low) and testing of electrolytes, kidney, and liver function. Frequently, a chest X-ray is performed to see if there is a collection of leukemia cells in an immune organ of the upper chest, called the thymus. In order to definitively establish the diagnosis of acute leukemia in children a test called a bone marrow examination is performed, in which an oncologist inserts a needle into the hip bone while the child is sedated. This involves two parts performed as one procedure: an aspirate (pulling liquid marrow out of the bone) and a biopsy (taking a small piece of solid marrow). At the same time, a sample of spinal fluid is usually obtained to check to see if leukemia cells are present in the central nervous system (CNS)—this is done by a lumbar puncture.
At this point, it is possible to determine what type of leukemia a child has—usually either ALL or AML.
Because leukemia is a "liquid" tumor and is present in the bloodstream, imaging tests like CT and MRI scans are not typically used for diagnosis or staging. Instead, certain characteristics of the patient, the clinical presentation, and the leukemia cells themselves are used to assign patients to a risk category, which in turn determines what treatment is needed.
In children with ALL, the following features are important in determining risk category:
AML treatment is determined predominately by the genetics of the leukemia cells (see above). As is true for patients with ALL, if AML is found in the CNS, increased intensity therapies are used.
For both types of acute leukemia, these features will enable the oncologist to categorize the patient into a specific risk group and subsequently select an initial treatment plan. Afterwards, the response to therapy is monitored closely, and measured by looking at the blood and the bone marrow. After the first phase of therapy (see below), tests are performed to determine if a child with leukemia is in remission and the quality of this remission. Based on this determination, treatment may or may not be altered. As with many types of childhood cancer, this response to therapy has major prognostic implications.
For both ALL and AML, chemotherapy is the major form of treatment, and is divided into different phases of therapy. The structure and intensity of these phases differs significantly. This chemotherapy is administered intravenously and/or orally (systemically) so it can attack the leukemia cells in the bloodstream and bone marrow. In addition, all patients receive some chemotherapy administered directly into the spinal fluid (intrathecal chemotherapy), even if no leukemia cells are found in the CNS at diagnosis. The drugs are given directly into the spinal fluid because most drugs that are given by mouth or vein do not travel well into the CNS. Placing chemotherapy into the spinal fluid serves to prevent the leukemia from coming back in the CNS. In certain cases, radiation to the brain will be administered, although the frequency with which radiation is used in childhood acute leukemia treatment is diminishing.
The initial treatment of acute leukemia involves not just appropriate chemotherapy, but management of complications that the leukemia cells cause, such as giving transfusions of red blood cells or platelets for low blood counts. Occasionally these complications constitute medical emergencies and can be life-threatening; sometimes these improve with prompt initiation of anti-leukemia therapy, but frequently additional measures are taken to ensure that these emergencies are not exacerbated early in the course of treatment.
Treatment for ALL is divided into several major phases—while the names and details of these phases vary depending on the specific regimen that the oncologist selects (and the risk group of the patient), the general structure is the same for almost all forms of ALL. The early phases of therapy are generally intensive (although still mostly outpatient), and are followed by a long period of low-intensity therapy called maintenance (or continuation) that is all outpatient and mostly oral (by mouth) chemotherapy.
In contrast to ALL, where treatment occurs over several years, AML is treated more briefly (over 6-8 months). Because AML is generally more difficult to cure than ALL, the treatment is intensive and associated with more side effects, particularly from infection. For this reason patients largely remain hospitalized during chemotherapy cycles. Treatment is divided into blocks which are 4-6 weeks long with brief breaks in between.
In certain situations, the oncologist may decide that a child's AML is not going to be successfully cured with conventional chemotherapy. This decision is based on the genetics of the leukemia cells and/or the individual's response to the initial phases of therapy. If this is the case, a bone marrow transplant (BMT) may be utilized. For AML, successful BMT utilizes a donor other than the patient (called allogeneic BMT), either a sibling, an unrelated volunteer donor, or unrelated umbilical cord blood. The oncologist will determine if the child has a match from any of these donor sources, based on special tests that determine the likelihood of a patient successfully accepting a transplant. Generally, if a patient has a sibling match, this is the preferred donor source; any given full sibling has an approximately 25% chance of being a match.
If BMT is the treatment of choice, this procedure will be performed after 3-4 cycles of intensive chemotherapy. A BMT is performed by administering very high doses of chemotherapy and/or radiation that serve the purpose of treating the AML and allowing for the successful acceptance of the donated blood/marrow, followed by infusion of the donated blood/marrow. BMT is associated with many risks—particularly infection and organ damage—and patients are monitored very closely in the hospital until it is felt that they are safe to go home (typically 4-6 weeks after transplant).
There is a special form of AML, called APML (or APL), that is treated very differently. Because this type of AML responds to medications targeted at very specific aspects of the cancer cells, it is easier to treat, using less intensive regimens.
The survival rate for childhood leukemia depends on what general type of leukemia (AML or ALL) a child has, and the specific features of the leukemia that determine the risk category. One of the most important markers for prognosis is response to therapy. It is important to remember that all risk assignment is treatment-based, meaning that a child with so-called "high risk ALL" may have the same chance of cure as a child with a lower risk assignment, but only if they receive more intensive therapy. Below is a very general review of the probability that treatment will be successful based on type of leukemia and risk group.
With current treatment approaches approximately 80-90% of childhood ALL will be cured. This differs by risk group:
Overall, approximately 60% of children with AML can be cured of their disease. This number is higher for those with so-called low-risk disease, but much lower for patients with high-risk features or those that do not respond well to initial therapy even with much more intensive therapy (that may include BMT).
While acute leukemia is by far the most common form of childhood leukemia, other types can occur. Of these, the most common is chronic myelogenous leukemia (CML). This type of leukemia is common in adults and presents in a different way than acute leukemia. Because the leukemia cells in CML almost all have a specific genetic change that responds very well to a specific drug (imatinib mesylate), this drug (or a similar one) is the treatment of choice for control of CML. Children with CML are treated with imatinib, but since this drug only controls the disease without achieving a true cure, more definitive therapy with BMT is sometimes needed.
While not technically classified as leukemia, children can develop myelodysplastic syndrome (MDS), which is a problem with the development and maturation of bone marrow cells that can progress to AML. These patients present with symptoms of low blood counts as well, but bone marrow testing does not reveal acute leukemia cells. These children require BMT for cure.
Hunger SP, Loh ML, Whitlock JA et al. Children's Oncology Group's 2013 Blueprint for Research: Acute Lymphoblastic Leukemia. Pediatric Blood & Cancer. 2012: 1-7.
Orkin Stuart et al. Oncology of Infancy and Childhood. Philadelphia: Saunders, 2009. Print.
Rubnitz JE. How I treat pediatric acute myeloid leukemia. Blood. 2012;119:5980-5988.
National Cancer Institute's Childhood Cancers Page
Childhood Leukemia Information from Children's Hospital of Philadelphia