Introduction

Chronic myelomonocytic leukemia (CMML) is a clonal bone marrow disorder of stem cells. The distinguishing feature is a persistent absolute monocytosis (blood monocytes greater than 1 X 109/L). Studies of the disease's natural history reveal heterogeneity in pathology, clinical features, and the outcome of untreated patients, as measured by duration of survival. The recent World Health Organization (WHO) reclassification of CMML and its inclusion as a separate entity in the groups of myeloproliferative/myelodysplastic disorders reflects two major pathologic features that appear with varying prominence in individual patients: cellular proliferation, which involves predominantly white blood cell (WBC) lineage and is often associated with organ infiltration and organomegaly, and maturation defects characterized by marrow dysplasia, which may involve all cell lineages, and result in anemia, thrombocytopenia, and rarely neutropenia.1 Either proliferative or dysplas-tic features may predominate in the clinicopathologic profile of individual patients, and the predominance may change during the course of the disease, usually from dysplastic to proliferative. Whether CMML should be categorized into dysplastic and proliferative entities based on the arbitrarily chosen "cutoff" for WBC at 13 X109/L, or whether these features could be viewed as various stages of the same disease, is still being discussed2 3 and was recently reviewed.3 It is tempting to speculate that different cellular regulatory pathways may be activated as a consequence of the presence of different primary or secondary molecular lesions. However, as will be discussed later in this chapter, these basic features do have some relevance in the current treatment of individual patients. These features are only marginally associated with prognosis and survival.34

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