A lineage switch in acute monocytic leukemia. A case report.

A case of congenital monocytic leukemia that underwent a lineage switch to acute lymphocytic leukemia (ALL) is described. The original leukemia had typical monocytic features, as evidenced by morphology (FAB M5), cytochemistry (nonspecific esterase) and immunophenotype (My4 positive). Cytogenetic study showed a pseudodiploid clone t(9;11)(p22;q21) that could be interpreted as a variant of the t(9;11)(p22;q23) reported in patients with the M5 type of leukemia. After successful remission induction with single-agent chemotherapy (VM-26) and subsequent sustained remission for 12 months with alternating VM-26 and VP-16-213, lineage switch to ALL (FAB L1) occurred. The presence of both lymphoid and myeloid markers on leukemic cells at lineage switch suggested the biphenotypic character of the patient's ALL. Our observation indicates that a lineage switch can occur from monocytic leukemia to ALL, although most of the cases previously reported have been in the reverse direction. This case emphasizes again the need to carry out careful and comprehensive marker studies to gain insight into the possible prognostic significance and the application of appropriate therapy.