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In the French–American–British (FAB) classification, chronic haematological malignancies were classified either as a myelodysplastic syndrome (MDS) or as a myeloproliferative disorder or disease (MPD). Chronic myelomonocytic leukaemia (CMML) was regarded as MDS, whereas atypical chronic myeloid leukaemia (aCML) was regarded as an MPD. This was never very satisfactory since, although CMML could have dysplastic features, this was equally true of aCML. This problem was resolved by the 2001 World Health Organization (WHO) classification which adopted the idea of a category for conditions with features overlapping between these two groups of neoplastic conditions, MDS/MPD [1] (Table 11.1). In the 2008 WHO classification the designation was changed to myelo -dysplastic/myeloproliferative neoplasms (MDS/MPN). This group includes only patients who have overlapping features at diagnosis. Patients with an MPN who develop dysplastic features as a result of disease evolution are not included. Similarly, patients with MDS who develop proliferative features as a result of disease acceleration or evolution are excluded. It is possible that some patients who have overlapping features at presentation may represent a disease in evolution from a previous MDS or MPN, but if this cannot be proven then they belong in this group. Other patients may have a condition with genuine overlapping features from disease initiation.

Table 11.1The myelodysplastic/myeloproliferative neoplasms

By definition, the WHO classification excludes patients from this group who have BCR-ABL1 fusion or isolated 5q-.

Juvenile myelomonocytic leukaemia (JMML) includes conditions that would previously have been categorized as juvenile myeloid leukaemia or as the monosomy 7 syndrome.

The unclassifiable group include a number of patients with refractory anaemia with ring sideroblasts and thrombocytosis (RARS-T) (Figures 11.1, 11.2, 11.3, 11.4), who often have a JAK2 V617F mutation [2]. Patients with a JAK2 mutation may form a discrete group. In around half of them the mutation is homozygous (due to mitotic recombination), differentiating this group of patients from those with essential thrombocythaemia [3]. Patients with a JAK2 mutation have a higher red cell count, higher white cell count (WBC) and better prognosis than those without a mutation [3]. Patients with MDS/MPN, unclassifiable, who lack a JAK2 mutation, form a more heterogeneous group than those with the mutation. Some of them have features typical of MDS but also have an elevated WBC.

Figure 11.1

Peripheral blood (PB) film of a patient with refractory anaemia with ring sideroblasts and thrombocytosis (RARS-T) showing macrocytosis, hypogranular platelets and a neutrophil demonstrating the acquired Pelger-Hüet anomaly (evidence of myelodysplasia) plus thrombocytosis with large platelets (evidence of myeloproliferation). MGG, high power.

Figure 11.2

PB film of a patient ...

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