Flow cytometric and cytomorphological definition of remission achievement in children with acute myeloid leukemia

Abstract

   The achievement of clinical and hematological remission at the end of induction therapy is one of the key treatment response parameters in pediatric acute myeloid leukemia (AML). Besides conventional cytomorphological evaluation of bone marrow (BM) blast count, minimal residual disease (MRD) measurement has been widely applied in routine clinical practice in recent years.   The aim of the study was to compare the results of flow cytometric MRD evaluation with the results of cytomorphological BM investigation when assessing the achievement of remission at the end of induction in children with AML.   The study was approved by the Independent Ethics Committee and the Scientific Council of the Dmitry Rogachev National Medical Research Center of Pediatric Hematology, Oncology and Immunology. We analyzed BM samples obtained from 402 children with AML, who had been treated according to the AML-MRD-2018 protocol and undergone simultaneous cytometric and cytomorphological BM investigation at the end of induction. A myelogram count was performed on 500 nucleated cells per BM smear. MRD was measured by 10-color flow cytometry with the 0.1 % cut-off for reliable MRD-positivity. The threshold of 5 % blasts was used as the criterion of complete remission (CR). Overall concordance of the two methods was 83.3 % for the CR status confirmation: in 335 out of 402 patients, the presence or absence of CR was stated using both techniques. Half of the 67 discordant samples were obtained from patients with a significant monocytic component of the leukemic population: 14 (20.9 %) with AML M4 and 20 (29.9 %) with AML M5. Among all FAB subtypes, the highest concordance rate was noted in patients with M1 variant (91.7 %), while the worst comparability – in children with megakarioblastic leukemia (M7 type, 72.7 %). Failure to achieve CR by cytomorphology did not influence the outcome of the patients who achieved CR as confirmed by immunophenotyping. At the same time, for flow cytometric BM investigation, achieving MRD negativity (< 0.1%) was the most significant favorable outcome predictor even at this rather early stage. Moreover, relapse incidence in children who were in CR but MRD positive (≥ 0.1 %) was higher than in patients who did not achieve CR at the end of induction according to flow cytometry (MRD ≥ 5 %), especially in the intermediate-risk group. This difference can be explained by more intensive chemotherapy (FLAI instead of HAM cycle) given to patients who did not achieve CR at the end of induction, and patients in the intermediate-risk group were additionally re-stratified to a high-risk group with subsequent hematopoietic stem cell transplantation. Flow cytometric and cytomorphological BM examination for the CR status confirmation at the end of induction in children with AML demonstrated a relatively high concordance rate (83.3 %). CR achievement by cytomorphology does not influence final outcome, although for the flow cytometry conventional threshold of 5 % also seems inadequate. We can assume that the modification of therapy is also required for patients with MRD ≥ 0,1 % at this stage of treatment.