Introduction

The category of myelodysplastic/myeloproliferative disorders (MDS/MPD) includes malignant hematopoietic proliferations which, at the time of their initial presentation, display features of both myelodysplastic syndromes and myeloproliferative disorders (Oscier, 1997; Jaffe et al., 2001). Cytopenias and dysplastic changes of any cell line may be seen, similar to the myelodysplastic syndromes. Elevated white blood cell counts, hypercellular marrows with fibrosis, and organomegaly, features more commonly associated with myeloproliferative disorders, may also be present. The presence of fibrosis alone in cases that are otherwise typical of myelodysplasia should not be placed in this category. The three best-defined mixed myeloproliferative and myelodysplastic syndromes are atypical chronic myeloid leukemia (atypical CML), chronic myelomonocytic leukemia (CMML), and juvenile myelomonocytic leukemia (JMML). Features that help in differentiating the chronic phase of CML from atypical CML and CMML are listed in Table 10.1.

Atypical chronic myeloid leukemia

Atypical CML (Bennett et al., 1994) is a Philadelphia chromosome-negative and BCR/ABL-negative proliferative disorder that affects elderly patients, with an apparent male predominance. Its incidence is <2 cases for every 100 cases of t(9;22), BCR/ABL-positive CML (Jaffe et al., 2001). Patients have some features of CML including splenomegaly, an elevated white blood cell count of predominantly granulocytic cells, and moderate anemia. The major characteristic which distinguishes atypical CML is dysgranulopoiesis, which is often severe. Moreover, atypical CML may have an initial presentation more typical of myelodysplasia with a low white blood cell count and normal to decreased platelet counts (Oscier, 1996).