Clinical course of a patient with Philadelphia chromosome–positive chronic myelogenous leukemia developing isolated central nervous system relapse while under dasatinib therapy. At the time of blast crisis, morphologic and flow cytometry studies evidenced 55% of myeloid blasts in the bone marrow (BM) with a CD33⁺CD13⁺CD10⁻CD20⁻CD45⁺ immunophenotype (not shown). BCR-ABL levels in BM were quantified by real-time polymerase chain reaction according to a described standard protocol.²  Gray arrow indicates intrathecal therapy with liposomal cytarabine (Depocyte, 50 mg); black arrows indicate intrathecal therapy with methotrexate (MTX, 12 mg) and cytarabine (ARA-C, 50 mg). Inset displays a May-Grünvald-Giemsa–stained cytospin slide of cerebrospinal fluid (CSF) showing myeloid blasts with a CD33⁺CD13⁺CD10⁻CD20⁻CD45⁺ at flow cytometry (not shown). At the time of documented leptomeningeal disease, the bone marrow examination was consistent with chronic-phase disease with a blast differential count of 4%. CHR indicates complete hematologic response; CCR, complete cytogenetic response; FLAG-IDA, fludarabine (30 mg/m² days 1-5), cytarabine (2 g/m² days 1-5), and idarubicin (10 mg/m² days 3-5) combination chemotherapy; Allo-BMT, allogeneic bone marrow transplantation; and MTX-ARA-C-IDA, systemic therapy with high-dose methotrexate (3.5 g/m² day 1) and cytarabine (2 g/m² days 2-3) plus idarubicin (8 mg/m² days 2-3).