Chronic myeloproliferative disorders (CPMDs) are characterized by the chronic proliferation of one or more of the three hematopoietic cell lines or by marrow stromal cells, in various proportions. Polycythemia vera (PV), idiopathic myelofibrosis (IMF), chronic myelogenous leukemia (CML), and essential thrombocytosis (ET) have been traditionally classified as "chronic myeloproliferative disorders."1 These disorders are believed to originate from a clonal transformation of a multipotent hematopoietic progenitor cell, resulting in an overproduction of one or more of the myeloid (i.e., granulo-cytic, erythroid, and megakaryocytic) lineages in the absence of a defined stimulus.2 In contrast to the ineffective erythropoiesis observed in the myelodysplastic syndromes, the proliferating cells show relatively normal maturation, with a resultant increase in granulo-cytes, red blood cells, and/or platelets in the peripheral blood. Other features shared by these disorders include splenomegaly and hepatomegaly from extramedullary hematopoiesis; marrow hypercellularity, megakaryocyte hyperplasia, and dysplasia; chromosomal abnormalities of chromosomes 1, 8, 9, 13, and 20; leukemic skin infiltrates, and spontaneous transformation into acute blast phase or bone marrow failure due to myelofi-brosis or ineffective erythropoiesis.3-6 The diagnosis of individual entities is therefore complicated by their overlapping features. The presence of the BCR/ABL fusion gene in association with characteristic morphologic and clinical findings permits an unequivocal diagnosis of CML. However, no specific chromosomal or molecular markers exist uniformly for the other conditions. Diagnosis of the other conditions is made on the basis of clinical, laboratory, and morphologic findings, which can be misleading. For example, approximately 10% of patients in one study of IMF actually had PV, while many patients initially diagnosed as ET had PV, instead.7-8

As CML is discussed in detail in Chapters 16-22, we will concentrate on the Philadephia (Ph)-chromosome-negative CMPDs.

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