Leukaemia is a cancer of the white blood cells. White blood cells help to fight infection.
There are two different types of white blood cell – lymphoid cells (also known as lymphocytes) and myeloid cells. Normally these cells repair and reproduce themselves in an orderly and controlled way. In leukaemia, however, the process gets out of control and the cells continue to divide but do not mature.
Acute myeloid leukaemia is an overproduction of immature myeloid cells, called myeloblasts or blast cells.
Immature myeloid cells fill up the bone marrow and stop it making healthy blood cells. As these cells are immature, they cannot work properly. This puts the child at increased risk of infection. Symptoms such as bruising and anaemia are caused by the bone marrow’s inability to make enough healthy red blood cells and platelets.
There are different sub-types of AML. The sub-types of AML are classified according to exactly which type of cell is affected, the stage of development (maturation) the cells are at, and whether the cells are differentiated. Cells that have started to show some of the features of myeloid cells are said to show differentiation. Cells which do not show signs of becoming a particular type of white blood cell are undifferentiated.
Knowing the sub-type of AML is important as it helps doctors to decide on the best treatment.
The most commonly used classification system for AML is the French-American-British (FAB) system:
- M0: acute myeloid leukaemia with minimal evidence of myeloid differentiation
- M1: acute myeloblastic leukaemia without maturation
- M2: acute myeloblastic leukaemia with maturation
- M3: acute promyelocytic leukaemia (APL)
- M4: acute myelomonocytic leukaemia
- M5: acute monocytic/monoblastic leukaemia
- M6: acute erythroleukaemia
- M7: acute megakaryoblastic leukaemia
Types M4 and M5 are the most frequent; M0 and M6 are both very rare.
Incidence of AML
Around 70 of new cases of childhood AML are diagnosed every year in the UK.
Incidence varies with age. The highest risk is in children aged less than two years; the risk in children aged two to nine years is lower and it then rises through the adolescent years.
More boys than girls develop AML, by a ratio of 5:4. The reason for this difference between the sexes is not known.
Children with AML have a less positive outlook than children with ALL, the more common form of childhood leukaemia.
Overall, around two thirds of children diagnosed with AML are cured.
Infants below the age of one year and children aged 10-14 years have lower survival than children diagnosed at intermediate ages.
Survival is highest for children with the M3 sub-type.
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