Myelomastocytic transformation in chronic myeloid leukemia blast phase: A case report.

Myelomastocytic leukemia (MML) presenting as a blast phase manifestation of Chronic Myeloid Leukemia (CML) is exceptionally rare, with limited documented cases in the literature. Understanding its distinct clinicopathologic features and treatment outcomes is crucial for optimal patient management. A 45-year-old male with a history of CML since 2016, previously treated with imatinib and dasatinib, presented after treatment interruption with leukocytosis (WBC 27.3 × 103/μL) and 58% circulating blasts showing metachromatic granulation. Bone marrow examination revealed 30% blast cells with strong CD117 and tryptase positivity. Flow cytometry identified two distinct populations: 7% myeloblasts and 27% immature myeloid cells with bright CD117 expression. BCR-ABL1 rearrangement was confirmed with a ratio of 112% (IS). The patient received combination therapy with standard "3 + 7" induction chemotherapy and dasatinib. Despite complications of febrile neutropenia, the post-induction bone marrow examination demonstrated achievement of complete morphologic remission. This case highlights the successful initial treatment of myelomastocytic transformation in CML blast phase using intensive combination therapy. The detailed morphologic, immunophenotypic and molecular characterization provides valuable insights into this rare entity, while the favorable initial response supports an aggressive treatment approach. Long-term follow-up and further studies are needed to establish optimal treatment strategies.