Acute myeloid leukemia (AML) with CBFB::MYH11 fusion is characterized by the clonal proliferation of monocytic and neutrophilic myeloid precursors and typically an abnormal eosinophilic component. Mastocytosis is an abnormal accumulation of clonal mast cells in organs or tissues and may not be detected when associated with another neoplasm. This article reports a rare case of AML with CBFB::MYH11 fusion with aberrant mast cells, in which the detection of mast cells occurred late, impacting the patient's first diagnosis and prognosis.

Clinical data were collected from medical records and laboratory test results were obtained from the Clinical Analysis Laboratory of the University Hospital information systems.

A 51-year-old male patient was forwarded to a university hospital with fatigue and exhaustion, night sweats, weight loss, loss of appetite, and loss of consciousness during physical effort. Laboratory exams showed: hemoglobin 6.5 g/dL, hematocrit 19.5%, leukocytes 42.770/μL with 43% blasts, and platelets 6.000/μL. The myelogram revealed blasts, immature myeloid cells, and aberrant eosinophils. Immunophenotypic analysis detected the presence of 44.6% blasts (CD34++, CD45+) committed (CD38+) to the myeloid lineage (MPO+, CD13++, CD33+, CD117+, HLA-DR+FR), 20.4% monocytic cells where 70% were mature monocytes (CD14+, CD300e+), 18% promonocytes (CD14+, CD300e-), and 12% monoblasts (CD14-, CD300e-), indicating acute myelomonocytic leukemia. The presence of mast cells (0.1%) was observed. Subsequently, genetic analysis and molecular biology confirmed AML with CBFB-MYH11 fusion and FLT3-D835 mutation. The treatment protocol chosen was ARA-C + Daunorubicin. One month after the beginning of induction chemotherapy, immunophenotyping of the bone marrow showed 0.6% myeloblasts with a phenotype like that found at diagnosis and 0.2% of mast cells (CD117+++), of which 53% have aberrant phenotype (CD2-/+FR, CD25+, CD64-/+, CD203c+, CD123+FR). The patient presented a serum tryptase dosage of 14.5 ug/L (reference value: 11 ug/L), and the KIT-D816 gene was not detected (performed on a blood sample). Six months after the initial diagnosis, the patient achieved remission by morphological and immunophenotyping analysis.

According to the latest classification of hematological tumors established by the WHO, some criteria must be met for the diagnosis of mastocytosis. Although the patient presented the presence of aberrant mast cells in the bone marrow expressing CD2 and CD25, no other criteria for the definition of mastocytosis were met. It is possible to suggest that the patient's diagnosis is systemic mastocytosis associated with a hematologic neoplasm, but more investigations will be necessary to conclude this diagnosis. Patients with mastocytosis and AHN have a worse prognosis, nevertheless, this patient achieved remission of AML with CBFB::MYH11 fusion after induction chemotherapy.

It is crucial that the diagnosis and monitoring of hematologic neoplasms be carried out in a multidisciplinary manner, combining clinical and laboratory diagnostics using various techniques, particularly flow cytometry and molecular biology. In this case, these techniques were essential for diagnosing the aberrant mast cells, defining the prognosis, and monitoring minimal residual disease.