In blast-phase chronic myelogenous leukemia (CML), the percentage of blasts is > 20% in the peripheral blood or bone marrow. The blasts have dispersed chromatin, variably prominent nucleoli, and a high nucleus-to-cytoplasm ratio and are thus morphologically indistinguishable from those seen in de novo acute leukemia. Given the stem-cell nature of CML, they can be of either myeloid, lymphoid, or multilineage phenotype. Phenotype of blastic transformation tends to correlate with the secondary cytogenetic abnormalities acquired in the process of transformation. The myeloid phenotype occurs in about 70–75% of patients, and the leukemic cells in these patients commonly harbor + 8, + 19, + 21, i(17), or additional Philadelphia chromosome. The lymphoid blast phase is less common and less consistently associated with any particular chromosomal aberrations.
By permission of the publisher. From Gorman R, Finiewicz K. In Atlas of Cancer . Edited by M Markman and M Kalaycio. Philadelphia, Current Medicine, 2002. Available at www.images.md.