Case Series: Unveiling mass-forming neoplastic extramedullary hematopoiesis in chronic myeloid leukemia

Abstract Introduction/Objective Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm characterized by BCR::ABL1 fusion and predominantly granulocytic proliferation. Three main phases of disease have previously been recognized: chronic, accelerated and blast phase, though accelerated phase has become controversial. During the chronic phase, leukemic cells exist predominantly in the peripheral blood and bone marrow; however, during blast phase, blasts can proliferate in the blood, bone marrow, or extramedullary tissues. Methods/Case Report We present two instances of cutaneous extramedullary hematopoiesis associated with CML at our institution. Results (if a Case Study enter NA) Case 1: A 34-year-old man previously diagnosed with CML presented with painful pustules on his right thigh. Biopsy revealed an infiltration of primarily left-shifted granulocytes, along with scattered megakaryocytes, and rare erythroid precursors. Immunohistochemical studies were consistent with maturing extramedullary hematopoiesis, devoid of sheets or clusters of blasts. Peripheral blood analysis revealed marked neutrophilic leukocytosis with basophilia and eosinophilia. Bone marrow examination depicted left-shifted granulocytic hyperplasia, megakaryocytic hyperplasia, and fewer than 5% blasts. Case 2: A 40-year-old man presented with fatigue, night sweats, weight loss, hepatosplenomegaly and palpable nodules on his left arm and thighs. The biopsy of the thigh nodule uncovered leukemic cells at different developmental stages, along with eosinophils, erythroid precursors, and megakaryocytes. Immunohistochemical staining confirmed their presence, with only a few blasts observed. Blood and bone marrow tests showed leukocytosis with blasts less than 5%, indicating CML in the chronic phase. Molecular analysis confirmed BCR-ABL1 fusion. Conclusion Mass-forming neoplastic extramedullary hematopoiesis in the skin during chronic phase of CML is a feature that is not well-documented in the literature. This entity shows immature granulocytic precursors, but does not exhibit an increase in blasts and therefore should not be called leukemia cutis or myeloid sarcoma. It is important to raise awareness of this entity to prevent misinterpretation as myeloid sarcoma and subsequent unnecessarily aggressive therapy.