Leukemia — The Basics
May 30th, 2010 by The Doc
Leukemia.
That one word carries more impact for a patient than nearly any other term in the medical lexicon. In years past, a diagnosis of leukemia was always associated with a grim prognosis. Even today, when temporary remission can be achieved in a significant number of patients, long-term survival rates (“cure”) are less than impressive. In fact (paradoxically) some of the chemotherapeutic agents used to treat leukemias and other cancers – cyclophosphamide and etoposide, for example – actually increase one’s risk for leukemia.
Leukemias are cancers of white blood cells (WBCs). That may seem a little ironic since WBCs are the guardians of our immune systems, and they’re supposed to protect us from things like cancer. But any cell that can divide is susceptible to the random genetic damage that can lead to malignant transformation.
Since WBCs are found in the bloodstream, the bone marrow, the lymph nodes, the spleen, the liver, and other organs, it isn’t surprising that all of these tissues can be compromised whenever leukemia develops. The manifestations of leukemia arise from suppression of normal blood cell production in the marrow and from organ infiltration by leukemic cells.
Risk Factors for Leukemia
The risk for developing leukemia is increased by a variety of factors:
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Exposure to ionizing radiation (e.g., citizens of Nagasaki and Hiroshima following atom bomb attacks; nuclear “down-winders” in Nevada and southern Utah; patients who have undergone radiotherapy for medical conditions)
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Chemical exposure (benzene, aniline dyes used in hair coloring, pesticides, herbicides)
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Smoking
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Viruses (human T-lymphotropic virus, Epstein-Barr virus)
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Immunodeficiency disorders
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Chromosomal disorders (Down syndrome, Fanconi’s anemia, etc.)
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Age (more than 70% of leukemias occur in individuals over 50)
Classification of Leukemias
Originally, leukemias were called “acute” or “chronic” based upon a patient’s life expectancy, but those lines have been blurred somewhat by increased survival times brought about by modern therapies. Nowadays, most experts further categorize leukemias according to the cell types involved and the cells’ level of maturation. Thus, acute leukemias are more likely to be associated with immature, poorly differentiated cells – cells that tend to divide more rapidly, spread more aggressively, and respond poorly to treatment. Chronic leukemias, on the other hand, typically exhibit more mature and better-differentiated cell types.
Leukemias are subdivided into the following cellular types: Acute leukemias are classified as lymphocytic (acute lymphocytic leukemia, or ALL) and myelocytic (acute myelogenous leukemia, or AML) types. Likewise, chronic leukemias are divided into CLL and CML types. Further categorization – useful for determining therapy and prognosis, particularly for acute leukemias – is based upon a FAB (French-American-British) system.
Treatments for Leukemia
Leukemias generally respond to some form of chemotherapy, and a small percentage of patients (around 5%) also undergo radiation treatment. Both forms of treatment are designed to damage the chromosomes of rapidly dividing or otherwise susceptible cells, thereby killing cancer cells without exerting undue damage on normal cells. Unfortunately, these treatments do not distinguish between cancer cells and normal cells that also divide rapidly – blood cells and platelets, the cells lining the gastrointestinal tract, hair follicles, etc. This “spillover” is what creates the harmful side effects of leukemia therapies.
Interferon, a cytokine that influences the immune system, is often used to treat chronic leukemias. Ostensibly, interferon limits the reproduction of leukemia cells while enhancing the immune system’s response to the cancer. Other cytokines have found similar use in treating different forms of leukemia.
Stem cell therapy will probably become an important adjunct for patients who have undergone chemotherapy or radiation to destroy their leukemia cells. Following such conventional treatment, stem cells can be reintroduced to a patient’s bone marrow to replace the destroyed cells and, hopefully, reestablish normal cell lines. This technology is new and carries its own burdens, but stem cells represent a promising modality for cancer therapy.
Growth factors, monoclonal antibody therapies, vaccines, and various immunomodulators are all finding a place in treating leukemias, but many of these modalities are still under investigation.