In 1951, Dameshek first speculated that different chronic proliferative disorders share similar clinical and hematologic features and that patients with one of these diseases often develop, during the course of their illness, symptoms more typical of another disease, usually more severe than the original one. He coined the term “myeloproliferative disorders” (MPD) for these, now widely recognized, clonal proliferations of an abnormal hematopoietic stem cell. This group of related diseases, characterized by a variable propensity to evolve into acute leukemia, included chronic myeloid leukemia (CML), polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF). Myeloid neoplasms that present with aberrant proliferative and dysplastic features have raised considerable controversy with respect to their classification. The classification of MPDs by the World Health Organization (WHO) in 2001 separated these diseases into several myelodysplastic/myeloproliferative groups, including juvenile myelomonocytic leukemia (JMML), chronic myelomonocytic leukemia (CMML), atypical CML, and a group of otherwise unclassifiable diseases. While JMML represents approximately 2 to 3% of leukemias in children, CMML and atypical CML are extremely rare in young people. Occasionally, CMML is diagnosed in an adolescent with persistent monocytosis, low blast count, and the absence of genetic features of JMML or CML. In addition, a CMML-like morphology accompanied by hepatosplenomegaly may be observed in secondary hematopoietic neoplasms, following chemo- or radiotherapy for a first cancer.
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