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WHO Classification of Tumours
Acute myeloid leukaemia, NOS
Tumours of haematopoietic and lymphoid tissues


The category of acute myeloid leukaemia, not otherwise specified (AML, NOS) encompasses those cases that do not fulfil criteria for inclusion in one of the previously described groups with recurrent genetic abnormalities, myelodysplasia-related changes or that are therapy-related. These tumours are felt to be derived from haematopoietic stem cells. The clinical relevance of some subgroups of AML, NOS is of questionable significance
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Arber DA, Stein AS, Carter NH, Ikle D, Forman SJ, Slovak ML (2003)
Prognostic impact of acute myeloid leukemia classification. Importance of detection of recurring cytogenetic abnormalities and multilineage dysplasia on survival.
Am J Clin Pathol 119: 672-80

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Tallman MS, Kim HT, Paietta E, Bennett JM, Dewald G, Cassileth PA, Wiernik PH, Rowe JM, (2004)
Acute monocytic leukemia (French-American-British classification M5) does not have a worse prognosis than other subtypes of acute myeloid leukemia: a report from the Eastern Cooperative Oncology Group.
J Clin Oncol 22: 1276-86

but they are retained in the classification because they define criteria for the diagnosis of AML across a diverse morphologic spectrum and include the unique diagnostic criteria for erythroleukaemia. Mutation analysis and cytogenetic studies are recommended for cases in this category and may offer more prognostic significance than the morphologic subtypes.

The primary basis for subclassification within this category is the morphological and cytochemical/immunophenotypic features of the leukaemic cells that indicate the major lineages involved and their degree of maturation. The defining criterion for AML is 20% or more myeloblasts in the peripheral blood (PB) or bone marrow (BM); the promonocytes in AML with monocytic differentiation are considered blast equivalents. The classification of acute erythroid leukaemia is unique and is based on the percentage of abnormal erythroblasts for pure erythroid leukaemia and the percentage of myeloblasts among non-erythroid cells for the erythroid/myeloid type.

It is recommended that the blast percentage in the BM be determined from a 500 cell differential count using an acceptable Romanowsky stain. In the PB, the differential should include 200 leukocytes; if there is a marked leukopenia, buffy coat smears can be used. Should an aspirate smear not be obtainable due to BM fibrosis and if the blasts express CD34, immunohistochemical detection of CD34 on biopsy sections may provide valuable information and may allow the diagnosis of AML if the 20% blast threshold is met. The major criteria required for this category are based on examination of BM aspirates, PB smears, and BM trephine biopsies. The recommendations for classification are applicable only to specimens obtained prior to chemotherapy. It should be noted that most of the epidemiologic data cited for each AML, NOS entity has been largely gathered from studies using the prior FAB classification scheme, and may not directly apply to series of patients classified by the WHO system, in which most patients will be classified into other, more specific entities, and for which sufficient epidemiologic data is not yet available.

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Acute myeloid leukaemia (AML) with myelodysplasia-related changes
Therapy-related myeloid neoplasms
Introduction: Myeloid Neoplasms