Leukaemia is a haematological neoplasm characterised by the overproduction of immature (blasts) or abnormally differentiated cells of the haematopoietic system in the bone marrow that often, but not always, extends into the peripheral blood.
This article aims to provide an overview of leukaemia as an entity and how it is classified. For a further detailed discussion, including radiographic features, please refer to the specific articles on each subtype. Also, for the entire classification of haematological neoplasms, please refer to the article on "WHO classification of tumours of haematopoietic and lymphoid tissues".
General classification
Leukaemia is classically divided according to the percentage of blasts in the bone marrow or peripheral blood:
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acute: when there is a proliferation of mostly immature/poorly differentiated cells (blasts) in the bone marrow (exceeding 20% of the bone marrow cell population)
clonal cells buildup crowds out the marrow in detriment of healthy blood lineage cells
disease becomes symptomatic early
chronic: when there is a proliferation of mostly mature but abnormal leucocytes (abnormal leucocytosis) with or without associated cytopenia
Specific classification
The 2016 revised WHO classification has an extensive list of subtypes of leukaemia, this article will list the most prevalent ones 4.
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acute lymphoblastic leukaemia (ALL)
commonly affecting children (~80% of leukaemias in this group) 3
usually severely symptomatic
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chronic lymphocytic leukaemia (CLL)
commonly affecting elderly patients (mainly over 75 years) 2
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commonly seen in adults, but is also the second most common form affecting children 2
male predominance
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chronic myeloid leukaemia (CML)
commonly affecting male adults
Philadelphia chromosome is present in 90% of cases 2
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