Leukemia is a cancer that affects blood cells and the stem cells in bone marrow that generate blood cells. There are four main types of leukemia, two acute and two chronic. All occur more often in men than women. Children's leukemias are almost always the acute types.
ALL starts in a type of bone marrow white blood cells called lymphocytes. Cancer occurs when the lymphocytes do not mature and instead reproduce rapidly. ALL invades the bloodstream quickly and spreads to other organs, preventing them from functioning properly. Without treatment, ALL spreads rapidly and can be fatal in a few months.
About 75% of childhood leukemia cases are ALL. Conversely, it is the least common leukemia among adults. The risk of ALL is highest among children aged 2 to 4 and gradually increases again after age 50.
Most patients have no known risk factors and the causes of ALL are not well understood. Known risk factors include radiation, exposure to benzene, infection with T-cell lymphoma virus (HTLV-1), and certain inherited conditions, including Down Syndrome (although most ALLs are not thought to have a strong genetic link). Smoking, workplace chemicals, electromagnetic fields, and hair dyes are suspected risk factors, but no definitive link has been established.
Symptoms of ALL are generalized and are caused by a shortage of white blood cells. They include weight loss, fever, night sweats, fatigue, and loss of appetite. Sometimes swelling of the liver and spleen can occur. ALL is diagnosed by blood testing.
The main treatment for ALL is chemotherapy. Surgery and radiation are rarely effective. High doses of chemotherapy drugs can destroy the stem cells that manufacture white blood cells, so sometimes stem cell transplants are required.
CLL starts in B lymphocytes in bone marrow. These bone-marrow cells mature somewhat more than in ALL, but not completely. They can perform some of the functions of white blood cells but are less effective, and gradually crowd out the fully formed white cells.
About one third of all leukemia cases are CLL. The disease mainly affects older adults, and the average age of diagnosis is 72. There are two types of CLL. One grows slowly and rarely needs to be treated. The other type grows faster and is a more serious disease.
As with ALL, there are few known risk factors. There seems to be some genetic component to CLL, as there is a greater likelihood of contracting the disease if a first-degree family member has it. CLL has the same symptoms and diagnostic methods as ALL. As with ALL, CLL is treated with chemotherapy. In addition, monoclonal antibodies may be used in some cases.
Acute myeloid leukemia starts in myeloid blood cells in bone marrow that would normally develop into white blood cells, red blood cells, or platelets. AML is really a group of related diseases with certain common characteristics. The different types of AML are caused by abnormalities in various chromosomes and are defined by which chromosome is abnormal.
AML is generally a disease of older people and is uncommon in people under 40. Most patients have no known risk factors. Smoking is the only proven lifestyle-related risk factor for AML. Exposure to benzene, certain chemotherapy drugs, radiation, and certain congenital conditions are also known causes.
The symptoms of AML are similar to those of ALL and diagnosis and treatment are the same.
CML starts in the blood-forming cells of the bone marrow and moves into the blood. The disease tends to build up over time and can eventually invade other parts of the body, often the spleen. CML can also develop into an acute leukemia.
CML normally affects adults and accounts for about 10% to 15% of all leukemias. As with other forms of leukemia, there are few, if any, risk factors aside from exposure to high-dose radiation. Symptoms are vague, similar to those described above, and can take years to develop.
Treatment for CML has advanced tremendously with the advent of a class of drugs known as tyrosine kinase inhibitors (TKIs), first introduced in 2001. Though it is too soon for scientific studies to show proven results, anecdotal evidence suggests a very good survival rate for at least five years. These drugs have become the standard treatment, although occasionally, chemotherapy and radiation might still be used.