How I (Diagnose and) Treat Myeloid / Lymphoid Neoplasms with Tyrosine Kinase Gene Fusions.

The fifth edition of the WHO classification and the International Consensus Classification (ICC) both include a category "myeloid/lymphoid neoplasms with eosinophilia and tyrosine kinase (TK) gene fusions" (WHO, MLN-TK; ICC, M/LN-eo-TK). This rare group comprises phenotypically and prognostically heterogeneous disorders that present a significant diagnostic challenge. The rapid and reliable identification of patients with MLN-TK may be delayed due to genetic complexity and significant phenotypic differences, which include chronic phase and primary/secondary blast phase (BP) of myeloid, or lymphoid or mixed phenotype in the bone marrow (BP-BM) and/or at extramedullary sites (extramedullary disease, EMD). As a result, the entire armamentarium of conventional molecular genetic and cytogenetic techniques complemented by modern sequencing technologies such as RNA sequencing or whole genome sequencing are often required to identify an underlying TK fusion. TK inhibitors (TKIs) with variable efficacy are available for all fusion genes, but a long-term favorable clinical course under TKI monotherapy is currently only observed in MLN-PDGFRA/PDGFRB fusion genes on imatinib. Since primary/secondary BP-BM/EMD occur more frequently in MLN-FGFR1/JAK2/FLT3/ETV6::ABL1, a sequential combination of selective TKIs with or without prior intensive chemotherapy, rarely local local radiationradiotherapy and/or subsequent allogeneic hematopoietic cell transplantation should be considered.