B-328 Acute Myeloid Leukemia Associated with Monoclonal Plasma Cells in a Child

Background Polyclonal bone marrow plasmacytosis is a well-described pathological process that, in rare cases, may exceed 20% of plasma cells (PC). It is associated with several conditions, such as infections, autoimmune diseases, liver cirrhosis, and paraneoplastic conditions in lymphomas and leukemias. Acute Myeloid Leukemia (AML) is a severe hematologic neoplasm characterized by an increase in immature cells in the Bone Marrow (BM) or peripheral blood. Monoclonal Gammopathy of Undetermined Significance (MGUS) is characterized by the presence of a monoclonal component <3g/dL and clonal PC <10% in the BM. Multiple Myeloma (MM) is characterized by BM plasmacytosis >10%, evidence of organ damage and a monoclonal component > 3g/dL. Rare cases of de novo AML with concomitant PC disease have been described. We describe a case of de novo AML with the presence of clonal anomalous PC presenting in a pediatric patient. Methods A 13-year-old male patient was diagnosed in January 2024 with pancytopenia (Hb 9.0g/dL, Ht 26.1%, WBC 15,960 cells/mm³, blasts 19%, neutrophils 11%, basophils 1%, lymphocytes 24%, monocytes 44%, platelets 77,000/mm³). BM examination revealed hypercellularity with 44.5% of monocytic blast, 5% PC with Russell Bodies and erythrocyte rouleaux. Immunophenotyping showed 63% monoblasts positive for CD45, CD34, CD117, CD13, CD33, HLA-DR, MPO, CD64, CD14, CD36, CD4, CD38, and 1.28% clonal kappa PC positive for CD38, CD138, CD45, CD117, CD19, CD28, CD27. BCR/ABL was undetectable, and total absence of metaphase in karyotype test. Serologies were negative, and laboratory results included: normal Lactate Dehydrogenase, albumin 3.2 g/dL, globulins 6.3 g/dL, creatinine 0.61 mg/dL, and ionic calcium 1.19 mmol/L. Measurable Residual Disease (MRD) was monitored from February to May 2024, initially showing a positive result for AML with the presence of clonal kappa PC, and on the follow up becoming negative for AML with the presence of polyclonal PC. Results It is suggested that cases of AML with reactive plasmacytosis may be related to the production of IL-6 by myeloblasts. Furthermore, cases of AML concomitant with MM have been described in the literature in adults and elderly patients. To our knowledge, this is the first reported case of a child with de novo AML and monoclonal PC with an aberrant/abnormal immunophenotype at diagnosis, but without meeting the diagnostic criteria for MM. During clinical follow-up, while MRD for AML remained positive, the clonal PC were replaced by polyclonal PC with a normal immunophenotype. Subsequently, MRD for AML could no longer be detected. Conclusion We describe a rare finding in pediatric patients of anomalous and clonal PC in de novo AML. Clinicians and pathologists should remain alert to the presence of these findings to differentiate clonal plasmacytosis from reactive plasmacytosis. Further studies are needed to explore new therapeutic strategies.