Morphologic Types of Blastic Transformation

As discussed earlier, CML originates in a pluripotent hematopoi-etic stem cell. Support for this hypothesis was initially derived from the observation that blastic transformation could result in a leukemic syndrome from one of virtually any hematopoietic lineage. Immature cells in patients having undergone blast transformation have included undifferentiated blasts, myeloblasts, promyelocytes, myelomonoblasts, monoblasts, erythroblasts, megakaryoblasts, or lymphoblasts (Table 3-4).93 Although morphology alone may be suggestive, the use of histochemical, ultrastructural, enzymatic, or immunologic studies allows for accurate classification of the precise subtype of blastic transformation. It is particularly important to exclude lymphoblastic blast crisis, due to the alternative and potentially less toxic therapeutic approach for patients with this type of disorder. The frequency of the phenotypic subgroups of blast crisis in 275 patients, reported by Greaves,93 is noted in Table 3-4. Approximately two-thirds of patients transform into a disease that represents acute myeloid leukemia, with Auer rod-containing blasts. Azurophilic cytoplasmic granules have been noted in immature cells obtained from patients with an apparent promyelocytic trans-formation.93 Serum lysozyme elevations may occur in patients with leukemias that resemble acute monocytic or acute myelomonocytic leukemia.94 Demonstration of myeloperoxi-dase at the ultrastructural level may be a clue to the identity of myeloblasts, which may otherwise be classified as undifferenti-ated blasts or lymphoblasts by routine light microscopy in histo-chemical stains.95 The use of cell surface marker studies and molecular biologic techniques now allow for more likely identification of the cell of origin. In the rare patient whose CML

Table 3-4. Morphological and Phenotypic Subgroups of

Blastic Crisis in Philadelphia-Positive CML

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