Acute myeloid leukemia (AML) is a clonal hematopoietic neoplasm due to acquired oncogenic mutations that disrupts differentiation, resulting in the accumulation of immature myeloid blasts in the marrow. The marrow replacement by blasts results in marrow failure and complications related to anemia, thrombocytopenia and neutropenia. AML can affect all ages, but the incidence increases throughout life.

In this case, the bone marrow aspirate shows blasts with monocytic and granulocytic differentiation, including eosinophilia with abnormal basophilic or “harlequin” granulation (WHO 2008: AML with myelomonocytic differentiation and eosinophila, FAB subtype: M4Eo). The particular finding of abnormal eosinophils with basophilic/”harlequin” granulation is associated with inv(16)(p13.1,q22) or t(16,16)(p13.1,q22) chromosomal abnormalities, both resulting in CBFB-MYH11 fusion. In the presence of a recognized CBFB-MYH11 fusion, acute myeloid leukemia may be diagnosed at blast counts less than 20%.

Patients with CBFB-MYH11 translocations have a relatively good prognosis, with improved response to high dose cytarabine. Those with KIT mutation (approximately 16% with exon 17 mutations, 13% with exon 8 mutations) have poor prognosis and higher risk of relapse, whereas those with +22 show a better prognosis.