Given currently available knowledge of the pathophysiology of hematological neoplasms, the MPDs are best classified under the rubric of myeloid neoplasms (Table 3-1). The bone marrow stem cell disorders can also be characterized according to the degree of cellular proliferation and whether there is increased production of mature or immature cells. It is also helpful to consider the balance that must exist between proliferation and differentiation in the stem cell compartment for hematopoiesis to continue unabated. If maturation is impaired, it is possible that self-renewal will take precedence and result in an increased percentage of immature nonfunctional stem cells. The expansion of the immature compartment may result in (a) the lack of mature progeny able to carry out the routine functional duties of the blood system; (b) an inability of residual normal clones to find space for growth; and (c) the elaboration of peptides or other factors that have a deleterious effect on the growth of residual normal clones. If the number of immature bone marrow cells, or blasts, exceeds 20 to 30 percent of the total number of bone marrow cells in a bone marrow aspirate, then the patient is considered to have acute leukemia5,39 (Chapter 25). Aplastic anemia and paroxysmal nocturnal hemoglobinuria are believed to represent, at least in some cases, a relatively complete failure of bone marrow stem cell function resulting in variable degrees of bone marrow hypoplasia.40,41 Myelodysplas-tic syndromes, which share the propensity of the MPD to transform into AML, are considered under a separate nosology.5-7 The MDS refer to a heterogeneous group of disorders characterized by ineffective and abnormal hematopoiesis with variable degrees of cytopenias. MDS, formerly termed dysmyelopoietic syndrome, refractory dysmyelopoietic anemia, smoldering leukemia, or pre-leukemia, have also been shown to originate from a multipotent hematopoietic stem cell precursor with the potential for differentiation.42 While pathology and histologic classification are useful, (Table 3-2) prognosis in MDS is determined by clinical features such as the number of cytopenias, the degree of marrow infiltration by blasts, and the cytogenetics.43

Table 3-1. WHO Classification of Myeloid Neoplasms

Myeloproliferative diseases (overproliferation) Chronic myelogenous leukemia, Philadelphia chromosome positive [t(9;22)(q34;q11), BCR/ABL] Chronic neutrophilic leukemia

Chronic neutrophilic leukemia/hypereosinophilic syndrome Chronic idiopathic myelofibrosis Polycythemia vera Essential thrombocythemia Myeloproliferative disease, unclassified Myelodysplastic/myeloproliferative diseases (overlap) Chronic myelomonocytic leukemia Atypical chronic myelogenous leukemia Juvenile myelomonocytic leukemia Myelodysplastic syndromes (impaired maturation) Refractory anemia With ringed sideroblasts Without ringed sideroblasts Refractory cytopenia (myelodysplastic syndrome) with multilineage dysplasia Refractory anemia (myelodysplastic syndrome) with excess blasts (5-20 percent marrow blasts) 5q- syndrome

Myelodysplastic syndrome (unclassified) Acute myeloid leukemias (both overproliferation and impaired maturation)

(From Harris et al.,5 with permission).

The MPDs, in contrast, represent an increased production of mature cells. As indicated above, except for CML, these diseases are best grouped according to clinical features, generally pertaining to the degree of cellular proliferation among the several cell lines (Table 3-3). The particular features of each of the MPDs will be discussed subsequently. Another unifying feature of the bone marrow stem cell disorders in general and the MPDs in particular, is the increased propensity of these conditions to transform to acute leukemia (Plate 3-1A). For example, virtually all patients with CML who do not undergo an allo-geneic bone marrow transplant will eventually undergo a blas-tic transformation.12 However, the likelihood of the conversion to acute leukemia is 10 percent or less in the other MPDs,44-46 but may be potentiated by the use of therapeutic agents that are mutagenic, such as chlorambucil.47

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