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What are the key statistics for chronic lymphocytic leukemia?

What is chronic lymphocytic leukemia?

Chronic lymphocytic leukemia (CLL) is a type of cancer that starts from white blood cells (called lymphocytes) in the bone marrow. It then invades the blood. Leukemia cells tend to build up in the body over time, but in many cases people don't have any symptoms for at least a few years. In time, it can also invade other parts of the body, including the lymph nodes, liver, and spleen. Compared to other types of leukemia, CLL usually grows slowly.

Doctors have found that there seem to be 2 different kinds of CLL:

One kind of CLL grows very slowly and rarely needs to be treated. People with this kind of CLL survive an average of 15 years or more.
The other kind of CLL grows faster and is a more serious disease. People with this form of CLL survive an average of about 8 years.

The leukemia cells from these 2 types look alike, but lab tests can tell the difference between them. The tests look for proteins called ZAP-70 and CD38. Patients whose CLL cells contain low amounts of ZAP-70 and CD38 have a better prognosis (outlook).

Leukemia is different from other types of cancer that start in organs like the lungs, colon, or breast and then spread to the bone marrow. Cancers that start elsewhere and then spread to the bone marrow are not leukemia.

Normal bone marrow, blood, and lymphoid tissue

To understand the different types of leukemia, it helps to know some basic facts about the blood and lymph systems.

Bone marrow

Bone marrow is the soft inner part of some bones such as the skull, shoulder blades, ribs, pelvis, and backbones. The bone marrow is made up of a small number of blood stem cells, more mature blood-forming cells, fat cells, and supporting tissues that help cells grow.

Blood stem cells go through a series of changes to make new blood cells. During this process, the cells develop into either lymphocytes (a kind of white blood cell) or other blood-forming cells. The blood-forming cells can develop into 1 of the 3 main types of blood cell components:

Red blood cells
White blood cells (other than lymphocytes)
Platelets

Red blood cells

Red blood cells carry oxygen from the lungs to all other tissues in the body, and take carbon dioxide back to the lungs to be removed. Anemia (having too few red blood cells in the body) typically causes a person to feel tired, weak, and short of breath because the body tissues are not getting enough oxygen.

Platelets

Platelets are actually cell fragments made by a type of bone marrow cell called the megakaryocyte. Platelets are important in plugging up holes in blood vessels caused by cuts or bruises. A shortage of platelets is called thrombocytopenia. A person with thrombocytopenia may bleed and bruise easily.

White blood cells

White blood cells help the body fight infections. Lymphocytes are one type of white blood cell. The other types of white blood cells are granulocytes (neutrophils, basophils, and eosinophils) and monocytes.

Lymphocytes: These are the main cells that make up lymphoid tissue, a major part of the immune system. Lymphoid tissue is found in lymph nodes, the thymus gland, the spleen, the tonsils and adenoids, and is scattered throughout the digestive and respiratory systems and the bone marrow.

Lymphocytes develop from cells called lymphoblasts to become mature, infection-fighting cells. The 2 main types of lymphocytes are known as B lymphocytes (B cells) and T lymphocytes (T cells).

B lymphocytes protect the body from invading germs by developing (maturing) into plasma cells, which make proteins called antibodies. The antibodies attach to the germs (bacteria, viruses, and fungi), which helps other white blood cells called granulocytes to recognize and destroy them. B lymphocytes are the cells that most often develop into chronic lymphocytic leukemia (CLL) cells.
T lymphocytes can recognize cells infected by viruses and directly destroy these cells.

Granulocytes: These are white blood cells that have granules in them. Granules are spots that can be seen under the microscope. They contain enzymes and other substances that can destroy germs, such as bacteria. The 3 types of granulocytes -- neutrophils, basophils, and eosinophils -- are distinguished by the size and color of their granules. Granulocytes develop from blood-forming cells called myeloblasts to become mature, infection-fighting cells.

Monocytes: These white blood cells, which are related to granulocytes, also are important in protecting the body against bacteria. They start in the bone marrow as blood-forming monoblasts and develop into mature monocytes. 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. Macrophages also help lymphocytes recognize germs and start making antibodies to fight them.

Any of the blood-forming or lymphoid cells from the bone marrow can turn into a leukemia cell. Once this change takes place, the leukemia cells fail to go through their normal process of maturing. Most leukemia cells may reproduce quickly, but often the problem is that they don't die when they should. They survive and build up in the bone marrow. Over time, these cells spill into the bloodstream and spread to other organs, where they can prevent other cells in the body from functioning normally.

Types of leukemia

Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the specific type of leukemia helps doctors better predict each patient's prognosis (outlook) and select the best treatment.

Acute leukemia versus chronic leukemia

The first factor in classifying leukemia is whether most of the abnormal cells are mature (look like normal white blood cells) or immature (look more like stem cells).

In acute leukemia, the bone marrow cells cannot mature properly. Immature leukemia cells continue to reproduce and build up. Without treatment, most patients with acute leukemia would live only a few months. Some types of acute leukemia respond well to treatment, and many patients can be cured. Other types of acute leukemia have a less favorable outlook.

In chronic leukemia, the cells can mature partly but not completely. These cells may look fairly normal, but they are not. They generally do not fight infection as well as normal white blood cells do. And they survive longer, build up, and crowd out normal cells. Chronic leukemias tend to develop over a longer period of time, and most patients can live for many years. But chronic leukemias are generally harder to cure than acute leukemias.

Myeloid leukemia versus lymphocytic leukemia

The second factor in classifying leukemia is the type of bone marrow cells that are affected.

Leukemias that start in early forms of myeloid cells -- white blood cells (other than lymphocytes), red blood cells, or platelet-making cells (megakaryocytes) -- are myeloid leukemias (also known as myelocytic, myelogenous, or non-lymphocytic leukemias).

If the cancer starts in lymphocytes, it is called lymphocytic leukemia (also known as lymphoid or lymphoblastic leukemia). Lymphomas are also cancers that start in lymphocytes. While lymphocytic leukemias develop from cells in the bone marrow, lymphomas develop from cells in lymph nodes or other organs.

By considering whether leukemias are acute or chronic, and whether they are myeloid or lymphocytic, they can be divided into 4 main types:

Acute myeloid (or myelogenous) leukemia (AML)
Chronic myeloid (or myelogenous) leukemia (CML)
Acute lymphocytic (or lymphoblastic) leukemia (ALL)
Chronic lymphocytic leukemia (CLL)

Rarer forms of lymphocytic leukemia

There is the common form of CLL (which starts in B lymphocytes), but there are some rare types of leukemia that share some features with CLL.

Prolymphocytic leukemia (PLL): This is a type of leukemia in which the cancer cells are similar to normal cells called prolymphocytes -- immature forms of B lymphocytes (B-PLL) or T lymphocytes (T-PLL). Both B-PLL and T-PLL tend to be more aggressive than the usual type of CLL. Most cases will respond to some form of treatment, but over time they tend to relapse. PLL may develop in someone who already has CLL (in which case it tends to be more aggressive), but it can also occur in people who have never had CLL.

Large granular lymphocyte (LGL) leukemia: This is another rare form of chronic leukemia. The cancer cells are large and have features of either T lymphocytes or natural killer (NK) cells (another type of lymphocyte). Most LGL leukemias are slow-growing, but a small number are more aggressive. Drugs that suppress the immune system may be helpful, but aggressive cases are very hard to treat.

Hairy cell leukemia (HCL): This is another cancer of lymphocytes that tends to progress slowly. It accounts for about 2% of all leukemias. The cancer cells are a type of B lymphocyte but are different from those seen in CLL. There are also important differences in symptoms and treatment. This type of leukemia gets its name from the way the cells look under the microscope -- they have fine projections on their surface that make them look "hairy." Treatment for HCL can be very effective and is described in the section called "How is chronic lymphocytic leukemia treated?"

The rest of this document focuses mainly on CLL in adults, with some limited information on hairy cell leukemia. For information on other types of leukemia in adults and children, please see our separate documents on these topics.