Myeloid disorders constitute a subgroup of hematological malignancies that is separate from lymphoid disorders. The World Health Organization system for classification of tumors of the hematopoietic system divides myeloid disorders into acute myeloid leukemia and chronic myeloid disorders based on the presence or absence, respectively, of acute myeloid leukemia-defining morphological and cytogenetic features including the presence of 20% or more myeloblasts in either the bone marrow or the peripheral blood. A recently proposed semimolecular classification system for chronic myeloid disorders recognizes 3 broad categories: the myelodysplastic syndrome, classic myeloproliferative disorders (MPD), and atypical MPD. Classic MPD includes polycythemia vera, essential thrombocythemia, myelofibrosis with myeloid metaplasia, and chronic myeloid leukemia. Both myelodysplastic syndrome and BCR/ABL-negative classic MPD were previously discussed as part of the current ongoing symposium on hematological malignancies. The current review focuses on the diagnosis and treatment of both molecularly defined and clinicopathologically assigned categories of atypical MPD: chronic myelomonocytic leukemia, juvenile myelomonocytic leukemia, chronic neutrophilic leukemia, chronic basophilic leukemia, chronic eosinophilic leukemia, idiopathic eosinophilia including hypereosinophilic syndrome, systemic mastocytosis, unclassified MPD, and eosinophilic/mast cell disorders associated with mutations of platelet-derived growth factor receptors α (PDGFRA) and β (PDGFRB), FGFR1, and KIT.
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