Initial (pretreatment) bone marrow and lymph node evaluation. (A-B) Hypercellular bone marrow showing myeloid proliferation with focal increase in eosinophils but no increase in blasts (hematoxylin and eosin [H&E] stain; 10× and 40× objective lenses, respectively). (C-D) Reticulin stain showing moderate increase in fibrosis (40× objective lens) (C) with no increase in CD34⁺ blasts (40× objective lens) (D). (E-G) CD117 immunostain demonstrates focal paratrabecular areas of spindled mast cells (40× objective lens) (E) with aberrant expression of CD25 (F) and CD123 (G) (40× objective lens). (H) Peripheral blood (PB) smear showing leukocytosis with dysgranulopoiesis and abnormal monocytes (Wright-Giemsa stain; 100× oil objective lens). (I-L) Excisional lymph node biopsy showing 2 phenotypically and geographically distinct blast populations (H&E stain; 40× objective lens) (I), with separate T-lymphoblastic differentiation (J: CD3 and K: TDT; 40× objective lens) and myeloblastic differentiation (myeloperoxidase [MPO] stain; 40× objective lens) (L).