RESUMO
A 5-year-old white girl had a white blood cell count of 80,000/cu mm and 82% lymphoblasts in the peripheral blood. Acute lymphocytic leukemia (ALL) was diagnosed, defined by typical morphology (FAB-L1), a positive reaction for terminal deoxynucleotidyl transferase (TdT) and characteristic surface antigens detected with lymphoid monoclonal antibodies. The patient's peripheral lymphoblast count fell rapidly with ALL therapy, but the WBC count began to rise unexpectedly on the sixth day of treatment, with 84% myeloblasts and monocytoid blasts. Malignant cells in the bone marrow showed FAB-M4 morphology and were no longer reactive with antibodies directed against TdT or common ALL antigen. However, the myeloblasts continued to react with some of the same monoclonal antibodies as the original leukemia cells, and in addition expressed new determinants detected by monoclonal antibody TA-1 and peanut agglutinin. The rapid dynamic evolution of the malignancy during the course of induction chemotherapy favors the existence of a stem cell capable of differentiation into both lymphoid and myeloid clones that are both independently and selectively sensitive to specific chemotherapeutic regimens.