Lineage switch in Philadelphia chromosome‐ positive acute lymphoblastic leukemia

Background. An 8‐year‐old boy, initially diagnosed with Philadelphia chromosome‐positive (Ph+) acute lymphoblastic leukemia (ALL) (French‐American‐British [FABI‐Ll), relapsed with Ph+ acute myelogenous leukemia [AML) (FAB‐M2) 21 months after successful ALL treatment with standard therapy. Methods. The initial ALL presentation and subsequent AML relapse were analyzed by conventional morphologic, cytochemical, immunophenotypic, and cytogenetic studies. Results. Molecular analysis based on the polymerase chain reaction identified the presence of a bcr‐I‐abl fusion transcript at initial ALL presentation, the completion of ALL therapy, and AML relapse. Conclusions. The cytogenetic and molecular results support a common clonal origin for this process. This is case of lineage switch in a Ph+ acute leukemia. This case thus illustrates a manifestation of heterogeneous lineage differentiation among Ph+ acute leukemias. Cancer 1994; 73:1526‐32.