Classification of childhood leukemia

Most types of cancers are assigned numbered stages to describe their extent in the body, based on the size of the tumor and how far the cancer has spread.

But leukemia is not staged like most other cancers. The cancer starts in the bone marrow and quickly spreads to the blood, so leukemia cells are already throughout the body. The major concern is whether the leukemia cells have also started to collect in other organs such as the liver, spleen, lymph nodes, or central nervous system (brain and spinal cord). For instance, if the cancer cells have spread to the central nervous system in large numbers, they can be seen under a microscope in samples of the cerebral spinal fluid (CSF), the fluid that surrounds the brain and spine. If that has happened, treatment will need to be more intense in order to kill these cells.

The most important factor for leukemias is figuring out the type (ALL vs. AML) and subtype of the leukemia. This is done by testing samples of the blood, bone marrow, and sometimes lymph nodes or CSF. Figuring out the exact type and subtype of leukemia is a complex process that can be hard even for doctors to understand. For more detailed information, please see Childhood Leukemia.

Acute lymphocytic leukemia (ALL)

There are 4 main subtypes of ALL, based on the type of lymphocyte the leukemia cells come from and how mature they are. They are:

• Early Pre-B cell
• Pre-B cell
• Mature B cell
• T cell

Acute myelogenous leukemia (AML)

AML has many subtypes, based on the type of cell involved and how mature it is. The subtypes of AML are classified by how they look under the microscope and on the changes in the genes or chromosomes in the leukemia cells.

Hybrid or mixed leukemias

These leukemias have cells with features of both ALL and AML when they are tested in the lab. In children, these leukemias are generally treated like ALL and respond to treatment like ALL.

Chronic myelogenous leukemia (CML)

Chronic myelogenous leukemia (CML) is often a slower-growing cancer of early (immature) myeloid bone marrow cells. CML is not common in children, but it can occur.

The course of CML is divided into 3 phases, based mainly on the number of immature white blood cells (blasts) that are seen in the blood or bone marrow. If the leukemia is not cured with treatment, it can progress to more advanced phases over time.

• Chronic phase
• Accelerated phase
• Blast phase

If you have questions about what phase your child's CML is in, be sure to have the doctor explain it to you.

Prognostic factors in childhood leukemia

Prognostic factors are important in helping the doctor decide whether to use standard treatment or more intense treatment. These factors refer to certain differences among patients that affect the response to treatment. Prognostic factors seem to be more important in ALL than in AML.

Prognostic factors for children with acute lymphocytic leukemia (ALL)

Children with ALL are often divided into standard-risk, high-risk, or very high-risk groups. The higher-risk groups are given more intense treatment. As a rule, children in lower-risk groups have a better outlook than those at very high risk.

There are many prognostic factors used in ALL, but these 2 are the most important. Keep in mind that many children with one or more poor factors can still be cured.

• Age: Children with B-cell ALL between the ages of 1 and 9 tend to do better. Children younger than 1 and children older than 10 years are at higher risk. The outlook in T-cell ALL isn't affected much by age.
• White blood cell count (WBC): Children who have very high white cell counts (greater than 50,000 cells per cubic millimeter) when the leukemia is found are at high risk and need more intense treatment.

Other factors that can affect risk include the subtype of ALL, the child’s gender and race, whether the leukemia has spread to certain organs, chromosome changes in the leukemia cells, and how quickly the leukemia responds to treatment.

Prognostic factors for children with AML

Prognostic factors do not seem quite as important for AML as they are for ALL. These factors can include the age of the child when the AML is found, the white blood cell count, the subtype of AML, certain changes in chromosomes, how the cancer cells look under a microscope, whether the leukemia is a result of treatment for another cancer, and how quickly the leukemia responds to treatment.