Chronic Myeloid Leukemia (CML)
Chronic myeloid leukemia (CML), also known as chronic myelogenous leukemia, is a type of cancer that starts in the blood-forming cells of the bone marrow and invades the blood. It is usually associated with a chromosome abnormality called the Philadelphia chromosome. In CML, 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, the cells can also invade other parts of the body, including the spleen. CML can also change into a fast-growing acute leukemia that invades almost any organ in the body.
Although most cases of CML occur in adults, it rarely occurs in children, too. Their treatment is the same as for adults.
Leukemia is different from other types of cancer that start in organs such as 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.
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.
The blood-forming cells come from blood stem cells. These stem cells only make new blood-forming cells, not other kinds of cells. (This makes them different from embryonic stem cells, which are formed in a developing fetus and can develop into most other cells in the body.)
Stem cells go through a series of changes. 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), or 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. Having too few red blood cells in the body (anemia) typically causes weakness, fatigue, and shortness 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 (thrombocytopenia) can cause easy bruising and bleeding.
White blood cells
White blood cells are important in defending the body against 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 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 it 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 types of lymphocytes are known as B lymphocytes (B cells) and T lymphocytes (T cells).
Monocytes, 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 are also important in helping lymphocytes to recognize germs and start making antibodies to fight them.
Acute leukemia versus chronic leukemia
The first factor to consider in classifying a patient's leukemia is whether most of the abnormal cells are mature (look like normal white blood cells) or immature (look more like stem cells).
Acute leukemia: In acute leukemia, the bone marrow cells cannot mature the way they should. Immature leukemia cells continue to reproduce and build up. Without treatment, most patients with acute leukemia would only live 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.
Chronic leukemia: In chronic leukemia, the cells can mature partly but not completely. These cells are not really normal, although they look more normal than the immature cells of acute leukemia. They generally do not fight infection as well as do normal white blood cells. And, of course, 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. However, chronic leukemias are generally harder to cure than acute leukemias.
Myeloid leukemia versus lymphocytic leukemia
The second factor to consider in classifying leukemia is the type of bone marrow cells that are affected.
Leukemias that start in early 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).
A leukemia that starts in the cells that become lymphocytes is called lymphocytic leukemia (also known as lymphoblastic or lymphoid leukemia).
By looking at whether they are acute or chronic and whether they are myeloid or lymphocytic, leukemias can be divided into the 4 main types:
Although most cases of CML occur in adults, it rarely occurs in children, too. Their treatment is the same as for adults.
Leukemia is different from other types of cancer that start in organs such as 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 have some basic knowledge about the blood and lymph systems. The information which follows is quite complex. It may prove helpful, but you don’t need to understand all of it to learn more about your leukemia.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.
The blood-forming cells come from blood stem cells. These stem cells only make new blood-forming cells, not other kinds of cells. (This makes them different from embryonic stem cells, which are formed in a developing fetus and can develop into most other cells in the body.)
Stem cells go through a series of changes. 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), or 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. Having too few red blood cells in the body (anemia) typically causes weakness, fatigue, and shortness 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 (thrombocytopenia) can cause easy bruising and bleeding.
White blood cells
White blood cells are important in defending the body against 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 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 it 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 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 antibodies. These antibodies attach to the germs, such as bacteria, viruses, and fungi. Once the germ has been coated in this way, other white blood cells called granulocytes can recognize and destroy it.
- T lymphocytes can recognize cells infected by viruses and directly destroy these cells.
Monocytes, 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 are also important in helping lymphocytes to recognize germs and start making antibodies to fight them.
How leukemia starts
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. Although leukemia cells may reproduce too quickly, in most cases the problem is that they don’t die when they should. They survive and accumulate, often crowding out normal bone marrow cells. This can lead to low counts of normal blood cells. Over time, leukemia cells spill into the bloodstream and spread to other organs, where they can prevent other cells in the body from working the way they should.Types of leukemia
Not all leukemias are the same. Leukemias are divided into 4 main types. Knowing the specific type of leukemia can help doctors better predict each patient's prognosis (outlook) and select the best treatment.Acute leukemia versus chronic leukemia
The first factor to consider in classifying a patient's leukemia is whether most of the abnormal cells are mature (look like normal white blood cells) or immature (look more like stem cells).
Acute leukemia: In acute leukemia, the bone marrow cells cannot mature the way they should. Immature leukemia cells continue to reproduce and build up. Without treatment, most patients with acute leukemia would only live 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.
Chronic leukemia: In chronic leukemia, the cells can mature partly but not completely. These cells are not really normal, although they look more normal than the immature cells of acute leukemia. They generally do not fight infection as well as do normal white blood cells. And, of course, 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. However, chronic leukemias are generally harder to cure than acute leukemias.
Myeloid leukemia versus lymphocytic leukemia
The second factor to consider in classifying leukemia is the type of bone marrow cells that are affected.
Leukemias that start in early 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).
A leukemia that starts in the cells that become lymphocytes is called lymphocytic leukemia (also known as lymphoblastic or lymphoid leukemia).
By looking at whether they are acute or chronic and whether they are myeloid or lymphocytic, leukemias can be divided into the 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)