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Overview of Leukemia
The leukemias are cancers of the WBCs involving bone marrow, circulating WBCs, and organs such as the spleen and lymph nodes.
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)
Prior treatment with certain antineoplastic drugs, particularly procarbazine, nitrosureas (cyclophosphamide, melphalan), and epipodophyllotoxins (etoposide, teniposide)
Infection with a virus (eg, human T-lymphotropic virus 1 and 2, Epstein-Barr virus)
Preexisting conditions, including immunodeficiency disorders, chronic myeloproliferative disorders, and chromosomal disorders (eg, Fanconi anemia, Bloom syndrome, ataxia-telangiectasia, Down syndrome, infantile X-linked agammaglobulinemia)
Malignant transformation usually occurs at the pluripotent stem cell level, although it sometimes involves a committed stem cell with more limited capacity for differentiation. Abnormal proliferation, clonal expansion, and diminished apoptosis (programmed cell death) lead to replacement of normal blood elements with malignant cells.
French-American-British (FAB) Classification of Acute Leukemias
Manifestations of leukemia are due to suppression of normal blood cell formation and organ infiltration by leukemic cells. Inhibitory factors produced by leukemic cells and replacement of marrow space may suppress normal hematopoiesis, with ensuing anemia, thrombocytopenia, and granulocytopenia. Organ infiltration results in enlargement of the liver, spleen, and lymph nodes, and occasionally, in kidney and gonadal involvement. Meningeal infiltration results in clinical features associated with increasing intracranial pressure (eg, cranial nerve palsies).
Findings at Diagnosis in the Most Common Leukemias
Leukemias were originally termed acute or chronic based on life expectancy but now are classified according to cellular maturity.
Acute leukemias (see Acute Leukemia Overview) consist of predominantly immature, poorly differentiated cells (usually blast forms). Acute leukemias are divided into lymphocytic (ALL—see Acute Lymphocytic Leukemia (ALL)) and myelogenous (AML—see Acute Myelogenous Leukemia (AML)) types, which may be further subdivided by the French-American-British (FAB) classification (see Table: French-American-British (FAB) Classification of Acute Leukemias).
Chronic leukemias have more mature cells than do acute leukemias. They usually manifest as abnormal leukocytosis with or without cytopenia in an otherwise asymptomatic person. Findings and management differ significantly between chronic lymphocytic leukemia (CLL—see Chronic Lymphocytic Leukemia (CLL)) and chronic myelogenous leukemia (CML—see Chronic Myelogenous Leukemia (CML)).
Myelodysplastic syndromes (see Myelodysplastic Syndrome) involve progressive bone marrow failure but with an insufficient proportion of blast cells (< 30%) for making a definite diagnosis of AML; 40 to 60% of cases evolve into AML.
A leukemoid reaction is marked granulocytic leukocytosis (ie, WBC > 30,000/μL) produced by normal bone marrow in response to systemic infection or cancer. Although not a neoplastic disorder, a leukemoid reaction with a very high WBC count may require testing to distinguish it from CML (see Chronic Myelogenous Leukemia (CML)).
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