Chronic eosinophilic leukaemia (CEL) designates a type of Philadelphia (Ph)-negative BCR-ABL1-negative chronic myeloid leukaemia in which eosinophils and their precursors constitute the dominant neoplastic cell. However, the causative mutation occurs at least at the level of a multipotent myeloid stem cell (and, not infrequently, at the level of a pluripotent lymphoid-myeloid stem cell) so that other lineages are also part of the neoplastic clone. The distinction from chronic myelomonocytic leukaemia with eosinophilia and atypical chronic myeloid leukaemia with eosinophilia is somewhat artificial. It is very important to recognize cytogenetic/molecular genetic sub-groups of CEL since their clinicopathological features and their prognoses differ. CEL was arbitrarily defined in the 2001 World Health Organization (WHO) classification , as a BCR-ABL1-negative haematological neoplasm in which the eosinophil count is at least 1.5 × 109/l, other causes of eosinophilia are excluded and there is some positive evidence that the process is neoplastic (e.g. clonal eosinophils or increased blast cells in blood or bone marrow). In the 2008 WHO classification, specific categories for CEL associated with rearrangement of PDGFRA, PDGFRB or FGFR1 are recognized, with remaining cases being designated 'CEL, not otherwise specified' [2, 3]. In the case of PDGFRA and FGFR1 rearrangement, neoplasms may be lymphoid as well as myeloid.
Clinical features may include those expected in a chronic myeloid leukaemia, such as hepatomegaly, splenomegaly and symptoms of anaemia. Other features, which are attributable more specifically to the eosinophil proliferation and release of eosinophil granule contents, include cardiac damage (myocardial and endocardial), pulmonary infiltration with cough and dyspnoea, gastrointestinal symptoms, neuropathy and vasculitis. Cases associated with PDGFRA rearrangement are predominantly male and there is also a male excess in cases associated with rearrangement of PDGFRB.
Haematological and pathological features
The blood film shows increased eosinophils, and may or may not show an increase of neutrophils, monocytes or basophils or the presence of blast cells [4, 5]. Eosinophils may be cytologically normal or abnormal (Figs 3.1, 3.2, 3.3). A bone marrow aspirate shows an increase of eosinophils and their precursors (Fig. 3.4). Charcot-Leyden crystals may be present in macrophages. In some patients there is an increase of blast cells (but these are, by definition, less than 20% of bone marrow and peripheral blood cells) or an increase of cells of other myeloid lineages, sometimes including mast cells (seen in association with both PDGFRA and PDGFRB rearrangement). Transformation to acute myeloid leukaemia can occur. In cases associated with FIP1L1-PDGFRA fusion there can also be transformation to T-lineage acute lymphoblastic leukaemia/lymphoblastic lymphoma  and in cases with t(4;22)(q12;q11) and BCR-PDGFRA both B and T lymphoblastic transformation can occur. One patient with FIP1L1-PDGFRA fusion who presented with simultaneous acute eosinophilic leukaemia and T-lymphoblastic lymphoma has been reported . In cases associated ...