Myelomastocytic transformation in chronic myeloid leukemia: A diagnostic puzzle in two patients following treatment interruption

Blast crisis in chronic myeloid leukemia (CML) with metachromatic granules poses a diagnostic challenge. We report two CML patients who defaulted therapy and presented in blast crisis (BC) with numerous blasts showing metachromatic granules. Flowcytometric immunophenotyping and immunohistochemistry aided in identifying immature mast cells (MC), while the absence of the KITD816V mutation helped to narrow down and support the diagnosis of myeloid BC with MC differentiation or myelomastocytic leukemia (MML). These cases emphasize the diagnostic difficulty when MML occurs in BC and highlight the need for further research and refined classification of these rare hematologic entities.