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Cancer starts when cells in the body start to grow out of control. Cells in nearly any part of the body can become cancer. To learn more about cancer and how it starts and spreads, see What Is Cancer? For information about the differences between childhood cancers and adult cancers, see Cancer in Children.

Leukemias are cancers that start in cells that would normally develop into different types of blood cells. Most often, leukemia starts in early forms of white blood cells, but some leukemias start in other blood cell types..

Types of leukemia in children

There are different types of leukemia, which are based mainly on: 

• If the leukemia is acute (fast growing) or chronic (slower growing)
• If the leukemia starts in myeloid cells or lymphoid cells

Knowing the specific type of leukemia a child has can help doctors better predict each child’s prognosis (outlook) and select the best treatment.

Acute leukemias

Most childhood leukemias are acute. These leukemias can progress quickly, and typically need to be treated right away. The main types of acute leukemia are:

• Acute lymphocytic (lymphoblastic) leukemia (ALL): About 3 out of 4 childhood leukemias are ALL. These leukemias start in early forms of white blood cells called lymphocytes.
• Acute myeloid leukemia (AML): This type of leukemia, also called acute myelogenous leukemia, acute myelocytic leukemia, or acute non-lymphocytic leukemia, accounts for most of the remaining cases of childhood leukemia. AML starts from the myeloid cells that normally form white blood cells (other than lymphocytes), red blood cells, or platelets.

Rarely, acute leukemias can have features of both ALL and AML. These are called mixed lineage leukemias, acute undifferentiated leukemias, or mixed phenotype acute leukemias (MPALs). In children, they are generally treated like ALL and usually respond to treatment like ALL.

Both ALL and AML have subtypes. These are described in Childhood Leukemia Subtypes.

Chronic leukemias

Chronic leukemias are rare in children. These leukemias tend to grow more slowly than acute leukemias, but they are also harder to cure. Chronic leukemias can be divided into 2 main types:

• Chronic myeloid leukemia (CML): Also called chronic myelogenous leukemia, CML is rare in children. Treatment is similar to that used for adults (see ??Treatment of Children With Chronic Myeloid Leukemia [CML]). For more detailed information on CML, see Leukemia--Chronic Myeloid.
• Chronic lymphocytic leukemia (CLL): This leukemia is extremely rare in children. For more information on CLL, see Leukemia--Chronic Lymphocytic.

Juvenile myelomonocytic leukemia (JMML)

This rare type of leukemia is neither chronic nor acute. It starts in myeloid cells, but it usually doesn’t grow as fast as AML or as slowly as CML. It occurs most often in young children (average age of 2 years). Symptoms can include pale skin, fever, cough, easy bruising or bleeding, trouble breathing (from too many white blood cells in the lungs), rash, and an enlarged spleen, liver, and lymph nodes. For info on treating JMML, see Treatment of Children With Juvenile Myelomonocytic Leukemia (JMML).

Normal bone marrow, blood, and lymph tissue

To understand leukemia, it helps to know about the bone marrow, blood, and lymph systems.

Bone marrow

Bone marrow is the soft inner part of certain bones. It is made up of blood-forming cells, fat cells, and supporting tissues. A small number of the blood-forming cells are blood stem cells.

Blood stem cells go through a series of changes to make new blood cells. During this process, the cells become either lymphocytes (a kind of white blood cell) or other blood-forming cells, which are types of myeloid cells. Myeloid cells can develop into red blood cells, white blood cells (other than lymphocytes), or platelets.

Red blood cells

Red blood cells (RBCs) carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed.

Platelets

Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in stopping bleeding by plugging up holes in blood vessels.

White blood cells

White blood cells (WBCs) help the body fight infections. There are different types of white blood cells:

• Lymphocytes are mature WBCs that develop from lymphoblasts, a type of blood-forming cell in the bone marrow. Lymphocytes are the main cells that make up lymph tissue, a major part of the immune system. Lymph tissue is found in the lymph nodes, thymus (a small organ behind the breast bone), spleen, tonsils and adenoids, and bone marrow. It is also scattered through the digestive system and respiratory system. There are 2 main types of lymphocytes: B cells and T cells. (ALL, the most common type of childhood leukemia, can start in either B cells or T cells.) For more information, see Childhood Leukemia Subtypes.
• Granulocytes are mature WBCs that develop from myeloblasts, a type of blood-forming cell in the bone marrow. Granulocytes have granules that show up as spots under the microscope. These granules contain enzymes and other substances that can destroy germs, such as bacteria. The 3 types of granulocytes – neutrophils, basophils, and eosinophils – are distinguished under the microscope by the size and color of their granules.
• Monocytes develop from blood-forming monoblasts in the bone marrow and are related to granulocytes. After circulating in the bloodstream for about a day, monocytes enter body tissues to become macrophages, which can destroy some germs by surrounding and digesting them.

Start and spread of leukemia

Leukemia starts in the bone marrow. The leukemia cells can build up there, crowding out normal cells. Most often, the leukemia cells spill into the bloodstream fairly quickly. Some types of leukemia can also spread to other parts of the body such as the lymph nodes, spleen, liver, central nervous system (the brain and spinal cord), testicles, or other organs.

Some other childhood cancers, such as neuroblastoma or rhabdomyosarcoma, start in other organs and can spread to bone marrow, but these cancers are not leukemia.