Myeloid Neoplasms

For the purposes of this chapter, myeloid neoplasms are defined as including clonal proliferations of erythrocyte, granulocyte (neutrophil, eosinophil, or basophil), monocyte (including histiocyte), megakaryocyte, and mast cell lineages (Fig. 3.7). In humans, myeloid neoplasms are subdivided into acute myeloid neoplasms, myeloproliferative neoplasms (MPNs) (formerly chronic myeloid neoplasms), and myelodysplastic syndromes (MDSs) (Fig. 3.1). Attempts are being made to adopt similar terminology in describing the subcategories of myeloid neoplasms in veterinary medicine, however, universal acceptance and use have not yet occurred. Identification of the neoplastic cell line is not difficult if the cells are well-differentiated. When this is not the case, cytochemical, immunocytochemical, immunohistochemical, flow cytometric and other molecular tests may be required to determine cell type. Treatment and prognosis vary with stage at diagnosis, cell lineage and degree of differentiation, amongst other factors, emphasizing the importance of an accurate diagnosis.

Typically with myeloid neoplasia, the bone marrow is hypercellular and neoplastic cells occupy a considerable portion of the marrow space at the expense of normal tissue. Myeloid neoplasia is most often associated with release of neoplastic cells into the peripheral blood, and may be accompanied by cytopenia(s) of non-neoplastic hemopoietic cell line(s). Morphologic features and numbers of blast cells in bone marrow and peripheral blood are important in determining the subtype; the time frame is less important as it is often unknown. Blast cells are large, immature cells of a certain lineage (e.g. erythroid, granulocytic, monocytic, megakaryocytic), with fine chromatin, one or more nucleoli, and dark staining cytoplasms (Fig. 3.8). The term acute refers to a relatively undifferentiated phenotype, high numbers of blast cells in the bone marrow, and a more rapidly deteriorating clinical course following diagnosis. In contrast, the chronic myeloid neoplasms or MPNs have a more differentiated phenotype, lower numbers of blast cells in the bone marrow, and a more protracted clinical course following diagnosis. Acute neoplasms have blast cells exceeding 30% of all nucleated bone marrow cells, while chronic neoplasms have <30% blasts in the bone marrow. (Note: Although the limit has been decreased to 20% blasts in human medicine, this change has not yet officially occurred in veterinary medicine.) The importance of obtaining good quality bone marrow samples for submission to a reference laboratory and evaluation by an experienced clinical pathologist cannot be overemphasized. Bone marrow should be sampled, not just when hemopoietic neoplasia is suspected, but any time an unexplained hematologic abnormality is identified (see Protocol Manual for complete description of the procedure and sample handling; also see Bone Marrow Aspiration Video).