Leukemia is cancer of the blood and develops in the bone marrow. The bone marrow is the soft, spongy center of the long bones that produces the three major blood cells: white blood cells to fight infection; red blood cells that carry oxygen; and platelets that help with blood clotting and stop bleeding. When a child has leukemia, the bone marrow, for an unknown reason, begins to make white blood cells that do not mature correctly, but continue to reproduce themselves.
Normal, healthy cells only reproduce when there is enough space for them to fit. The body can regulate the production of cells by sending signals when to stop. With leukemia, these cells do not respond to the signals to stop and reproduce, regardless of space available.
These abnormal cells reproduce very quickly and do not function as healthy white blood cells to help fight infection. When the immature white blood cells, called blasts, begin to crowd out other healthy cells in the bone marrow, the child experiences the symptoms of leukemia (e.g. infections, anemia, bleeding).
Leukemia is the most common form of cancer in childhood. It affects approximately 3,800 children each year in the U.S., accounting for about 30% of childhood cancers.
Types of Leukemia
There are three main types of leukemia, including the following:
Acute Lymphocytic Leukemia (ALL)
ALL, also called lymphoblastic or lymphoid, accounts for about 75–80% of the childhood leukemias. In this form of the disease, the lymphocyte cell line is affected. The lymphocytes normally fight infection. With acute lymphocytic leukemia, the bone marrow makes too many of these lymphocytes and they do not mature correctly. The lymphocytes overproduce, thus, crowding out other blood cells. Immature blood cells (blasts) do not work properly to fight infection. Acute leukemia can occur over a short period of days to weeks. Chromosome abnormalities (extra chromosomes and structural changes in the chromosome material) are present in the majority of ALL patients.
Acute Myelogenous Leukemia (AML)
AML, also called granulocytic, myelocytic, myeloblastic or myeloid, accounts for about 20% of the childhood leukemias. Acute myelogenous leukemia is a cancer of the blood in which too many granulocytes, a type of white blood cell, are produced in the marrow. The granulocytes normally fight infection. With acute myelogenous leukemia, the bone marrow makes too many of these cells and they do not mature correctly. The granulocytes overproduce, thus, crowding out other blood cells. Immature blood cells (blasts) do not work properly to fight infection. Acute leukemia can occur over a short period of days to weeks. Children with certain genetic syndromes, including Fanconi anemia, Bloom syndrome, Kostmann syndrome, and Down syndrome, are at a higher risk of developing AML than other children.
Chronic Myelogenous Leukemia (CML)
CML is uncommon in children. Chronic myelogenous leukemia is cancer of the blood in which too many granulocytes, a type of white blood cell, are produced in the marrow. The granulocytes normally fight infection. With this disease, the bone marrow makes too many of these cells and they do not mature correctly. The marrow continues to produce these abnormal cells which crowd out other healthy blood cells. Chronic myelogenous leukemia can occur over a period of months or years. A specific chromosome rearrangement is found in patients with CML. Part of chromosome leukemia1.asp#9 breaks off and attaches itself to chromosome leukemia1.asp#22, so that there is an exchange of genetic material between these two chromosomes. This rearrangement changes the position and functions of certain genes, which results in uncontrolled cell growth. Other chromosome abnormalities can also be present.
The difference between lymphocytic and myelogenous is the stage of development of what is called the pluripotent stem cell. The pluripotent stem cell is the first stage of development of all of the blood cells (white blood cells, red blood cells and platelets). This stem cell goes through stages of development until it matures into a functioning cell. The type of leukemia is determined by where the cell is in the stage of development when it becomes malignant, or cancerous.
The stem cell matures into either the lymphoid or myeloid cells. The lymphoid cells mature into either B-lymphocytes or T-lymphocytes. If the leukemia is among these cells, it is called acute lymphocytic leukemia (ALL). If the leukemia is found even further along this stage of development, it can be further classified as B-cell ALL or T-cell ALL. The more mature the cell, the more difficult it is to treat.
The myeloid cells develop into platelets, red blood cells and specialized white blood cells called neutrophils and macrophages. There are many classifications of AML. The type of leukemia is determined by the stage of development when the normal cells become leukemia cells.
What Causes Leukemia?
The majority of childhood leukemias are acquired genetic diseases. This means that gene mutations and chromosome abnormalities in cells occur sporadically (by chance).
The immune system plays an important role in protecting the body from diseases, and possibly cancer. An alteration or defect in the immune system may increase the risk of developing leukemia. Factors such as exposure to certain viruses, environmental factors, chemical exposures and various infections have been associated with damage to the immune system.
With the exception of specific genetic syndromes, little is known about the causes of childhood leukemia.
How Is Leukemia Diagnosed?
In addition to a complete medical history and physical examination, diagnostic procedures for leukemia may include:
- Bone Marrow Aspiration and Biopsy: Marrow may be removed by aspiration or a needle biopsy under local anesthesia. In aspiration, a fluid specimen is removed from the bone marrow. In a needle biopsy, a small core of bone marrow cells is removed. These methods are often used together.
- Complete Blood Count (CBC): A measurement of size, number and maturity of different blood cells in a specific volume of blood.
- Additional Blood Tests: May include blood chemistries, evaluation of liver and kidney functions, and genetic studies.
- Computed Tomography Scan (CT or CAT Scan): A diagnostic imaging procedure (not usually required) that uses a combination of x-rays and computer technology to produce cross-sectional images (often called slices), both horizontally and vertically, of the body. A CT scan shows detailed images of any part of the body, including the bones, muscles, fat and organs. CT scans are more detailed than general x-rays.
- Magnetic Resonance Imaging (MRI): A diagnostic procedure (not usually required) that uses a combination of large magnets, radiofrequencies and a computer to produce detailed images of organs and structures within the body.
- X-ray: A diagnostic test which uses invisible electromagnetic energy beams to produce images of internal tissues, bones and organs onto film.
- Ultrasound (also called sonography): A diagnostic imaging technique (not usually required) which uses high-frequency sound waves and a computer to create images of blood vessels, tissues and organs. Ultrasounds are used to view internal organs as they function, and to assess blood flow through various vessels.
- Lymph Node Biopsy
- Spinal Tap/Lumbar Puncture: A special needle is placed into the lower back, into the spinal canal. This is the area around the spinal cord. The pressure in the spinal canal and brain can then be measured. A small amount of cerebral spinal fluid (CSF) can be removed and sent for testing to determine if leukemic cells are present. CSF is the fluid that bathes your child's brain and spinal cord.
What Are the Symptoms of Leukemia?
Because leukemia is cancer of the blood-forming tissue called the bone marrow, the initial symptoms are often related to irregular bone marrow function. The bone marrow is responsible for storing and producing about 95% of the body's blood cells, including the red blood cells, white blood cells and platelets.
When leukemia occurs, the abnormal white blood cells (blasts) begin to reproduce very rapidly and begin crowding out and competing for nutrients and space with the other healthy cells. The following are the most common symptoms of leukemia. However, each child may experience symptoms differently. Symptoms may include:
When red blood cells are unable to be produced because of the crowding in the marrow, anemia is present. With anemia, the child may appear tired, pale and may breathe faster to compensate for the decrease in oxygen carrying capacity. The number of red blood cells in a blood count will be below normal.
Bleeding and/or Bruising
When platelets are unable to be produced because of the crowding in the marrow, bleeding can occur and the child may begin to bruise more easily. Petechia are tiny red dots often seen on the skin of a child with a low number of platelets. Petechia are very small blood vessels that have "leaked" or bled. The number of platelets on a blood count will be below normal. Thrombocytopenia is the term used for a decreased number of platelets.