Leukemia is cancer of blood-forming tissue such as the bone marrow. Types of leukemia are grouped by the type of cell affected and by the rate of cell growth. Leukemia is either acute or chronic.
There are many different types of leukemia. Some are fast-growing and aggressive, others are slower to progress. Leukemias are also classified based on the type of blood cells affected. The most common types of leukemia include:
- Acute leukemia involves an overgrowth of very immature blood cells, also known as blasts. This condition is life-threatening because there are not enough mature blood cells to prevent anemia, infection and bleeding. A diagnosis of acute leukemia is made when there are 20 percent or more blasts in the bone marrow. Acute lymphoblastic leukemia (ALL) is most common during childhood and in early adulthood, although it is also diagnosed in adults 30 years old and older. Acute myelogenous leukemia (AML) occurs more often in adults.
- Chronic leukemia involves an overgrowth of mature blood cells. Usually, people with chronic leukemia have enough mature blood cells to prevent serious bleeding and infection. Chronic leukemia is more common in people between ages 40 and 70 and is rare among younger people.
- Myelodysplastic Syndrome (MDS) is a condition in which the bone marrow does not function normally and does not produce enough normal blood cells. The blood cells affected are white blood cells, red blood cells and platelets. Some cases of MDS may progress to acute leukemia, over time. MDS is most often found in patients nearing their 60s and 70s.
The different types of leukemia share some common symptoms, including:
- Persistent fatigue or feeling of weakness
- Loss of appetite
- Unintentional weight loss
- Easy bruising or bleeding
- Shortness of breath
- Petechiae (tiny red spots under the skin caused by bleeding)
Symptoms of acute lymphoblastic leukemia may also include the presence of painless lumps under the skin in the groin, underarm or neck, and/or pain under the ribs.
The specific cause of leukemia is still not known. Scientists suspect that viral, genetic, environmental or immunologic factors may be involved.
- Viruses: Some viruses cause leukemia in animals, but in humans, viruses cause only one rare type of leukemia. Even if a virus is involved, leukemia is not contagious. It cannot spread from one person to another. There is no increased occurrence of leukemia among people such as friends, family and caregivers who have close contact with leukemia patients.
- Genetics: There may be a genetic predisposition to leukemia. There are rare families where people born with chromosome damage may have genes that increase their chances of developing leukemia.
- Environmental factors, such as high-dose radiation and exposure to certain toxic chemicals, have been directly related to leukemia. But this has been true only in extreme cases, such as atomic bomb survivors in Nagasaki and Hiroshima or industrial workers exposed to benzene. Exposure to ordinary x-rays, like chest x-rays, is not believed to be dangerous.
People with immune-system deficiencies appear to be at greater risk for cancer because of the body’s decreased ability to resist foreign cells. There is evidence that patients treated for other types of cancer with some types of chemotherapy and/or high-dose radiation therapy may later develop leukemia.
All of these factors may explain why a small number of people develop leukemia. But, among most people, the cause of leukemia is not known.