Hematological Neoplasms with Eosinophilia

Simple Summary The diagnostic assessment of eosinophilias is complex, requiring a multidisciplinary approach and often involving diagnostic challenges. This work aims for a better understanding of the cytomorphological features, immunophenotype, and biological activity of the eosinophil. Additionally, the concepts of peripheral and bone marrow eosinophilia and their potential causes are reviewed. Finally, the review focuses on the broad differential diagnosis of hematologic diseases that may underlie eosinophilia and how to diagnose them. Among the findings that should raise suspicion of hematologic diseases associated with eosinophilia are the presence of splenomegaly and/or lymphadenopathy or an abnormal blood count. In recent years, with advances in molecular techniques, new hematologic malignancies such as myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions are being defined, where eosinophilia can serve as a guiding sign. In these cases, an accurate diagnosis allows for the use of targeted therapy with an improvement in the quality of life and survival of patients.Abstract Eosinophils in peripheral blood account for 0.3-5% of leukocytes, which is equivalent to 0.05-0.5 x 109/L. A count above 0.5 x 109/L is considered to indicate eosinophilia, while a count equal to or above 1.5 x 109/L is defined as hypereosinophilia. In bone marrow aspirate, eosinophilia is considered when eosinophils make up more than 6% of the total nuclear cells. In daily clinical practice, the most common causes of reactive eosinophilia are non-hematologic, whether they are non-neoplastic (allergic diseases, drugs, infections, or immunological diseases) or neoplastic (solid tumors). Eosinophilia that is associated with a hematological malignancy may be reactive or secondary to the production of eosinophilopoietic cytokines, and this is mainly seen in lymphoid neoplasms (Hodgkin lymphoma, mature T-cell neoplasms, lymphocytic variant of hypereosinophilic syndrome, and B-acute lymphoblastic leukemia/lymphoma). Eosinophilia that is associated with a hematological malignancy may also be neoplastic or primary, derived from the malignant clone, usually in myeloid neoplasms or with its origin in stem cells (myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase gene fusions, acute myeloid leukemia with core binding factor translocations, mastocytosis, myeloproliferative neoplasms, myelodysplastic/myeloproliferative neoplasms, and myelodysplastic neoplasms). There are no concrete data in standardized cytological and cytometric procedures that could predict whether eosinophilia is reactive or clonal. The verification is usually indirect, based on the categorization of the accompanying hematologic malignancy. This review focuses on the broad differential diagnosis of hematological malignancies with eosinophilia.