Leukemia is a malignant condition involving the excess production of immature or abnormal leukocytes, which eventually suppresses the production of normal blood cells and results in symptoms related to cytopenias.
Malignant transformation usually occurs at the pluripotent stem cell level, although it sometimes involves a committed stem cell with more limited capacity for self-renewal. Abnormal proliferation, clonal expansion, aberrant differentiation, and diminished apoptosis (programmed cell death) lead to replacement of normal blood elements with malignant cells.
The American Cancer Society estimates that in the United States in 2020 there will be about 60,530 new cases of leukemia (of all types) in adults and children, and about 23,100 deaths.
The current approach to classifying leukemia is based on the 2016 World Health Organization (WHO) system (classification for hematopoietic neoplasms). The WHO classification is based on a combination of clinical, morphologic, immunophenotypic, and genetic features. Other less commonly used classification systems include the French-American-British (FAB) system, which is based on the morphology of the abnormal leukocytes.
Leukemias are commonly also categorized as
The four most common leukemias and their distinguishing features are summarized in the table Findings at Diagnosis in the Most Common Leukemias.
For 2020, the American Cancer Society estimated the distribution of new US cases by leukemia type as follows:
Findings at Diagnosis in the Most Common Leukemias
Myelodysplastic syndromes are a group of clonal hematopoietic stem cell disorders unified by the presence of distinct mutations of hematopoietic stem cells. They involve progressive bone marrow failure but with an insufficient proportion of blast cells (< 20%) for making a definite diagnosis of acute myeloid leukemia; 40 to 60% of cases evolve into acute myeloid leukemia.
A leukemoid reaction is a neutrophil count > 50,000/mcL (> 50 × 109/L) not caused by malignant transformation of a hematopoietic stem cell. It can result from a variety of causes, particularly other cancers or systemic infection. Usually the cause is apparent, but apparent benign neutrophilia can be mimicked by chronic neutrophilic leukemia or chronic myeloid leukemia.
Risk of developing leukemia is increased in patients with
History of exposure to ionizing radiation (eg, post–atom bomb in Nagasaki and Hiroshima) or to chemicals (eg, benzene, some pesticides, polyaromatic hydrocarbons in tobacco smoke); exposure can lead to acute leukemias
Prior treatment with certain antineoplastic drugs, including alkylating agents, topoisomerase II inhibitors, hydroxyurea, and maintenance lenalidomide after autologous stem cell transplantation with melphalan-containing conditioning regimens for multiple myeloma; can lead to a type of acute myeloid leukemia called t-AML or therapy-related AML
Infection with a virus (eg, human T lymphotropic virus 1 and 2, Epstein Barr virus) can rarely cause certain forms of ALL; this is seen mainly in regions where such infections are common, such as Asia and Africa
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