Chronic eosinophilic leukemia (CEL)

Abstract:
Chronic eosinophilic leukemia (CEL) is part of the chronic myeloproliferative neoplasms. Its cause is an autonomous, clonal proliferation of eosinophilic precursor cells. There is an increase of eosinophils in the blood, bone marow and partly in some organs. Occasionally, a myeloproliferative syndrome is suspected, but there is no evidence for clonality or an increase of blasts. These cases are assigend to the hypereosinophilic syndromes (HES). If a BCR-ABL fusion gene is detected, the diagnosis is chronic myelocytic leukemia.

Clinical picture:
Damage of organs may occur by infiltration of organs and release of substances such as cytokines, enzymes, or other proteins. Especially feared is cardiomyopathy with impairment of heart valves, that occasionally leads to life threatening situations. It is impossible to predict whether organ damage occurs or not.

Hematology:

The eosinophils in the blood are most of the time clearly increased. By defnition, they must be >1.5 x10⁹/L. Generally, the cells are cytologically inconspicuous. Blasts may be present but account for less than <20%.

Bone marrow:
Eosinophils and eosinophilic precursors are increased and mostly show no morphological abnormalities. Blasts may occur but account for less than <20%.

Diagnosis:
According to the WHO classification , CEL can be diagnosed if

• eosinophils in the blood are >1.5 x 10⁹/L
• clonality of eosinophils is proven or
• blasts are increased (>2% in the blood, >5% in the bone marrow).
• Blasts in the blood or bone marrow may not exceed 20% .
• Polycythemia vera (PV), essential thrombocythemia (ET), primary myelofibrosis (PMF) , chronic myelomonocytic leukemia or atypical CML must be excluded.
• Inversion (16), BCR-ABL fusion gene or rearrangements of PDGFRalpha, PDGFRbeta or FGFR1 may not be present.

Myeloicytic and lymphocytic neoplasms with eosinophilia and evidence of rearrangements of PDGFRalpha, PDGFRbeta or FGF1 are assigned to another category of myelocytic neoplasms accoording to WHO classification .

The most frequent form is caused by the FIP1L1-PDGFRalpha fusion gene and can be proven by means of PCR or FISH. Diseases with rearranged PDGFRalpha or PDGFRbeta are of interest since they respond well to the tyrosine kinase inhibitor Imatinib (Gleevec^®).