Phenotypic change of acute monocytic leukemia to acute lymphoblastic leukemia on therapy.

Acute nonlymphocytic leukemias comprise most of the therapy-linked leukemias in cancer patients. We report here the unusual occurrence of acute lymphoblastic leukemia in a patient while on therapy for acute monocytic leukemia. The morphologic and histochemical studies were distinctly different between the initial presentation and the subsequent "relapse". At "relapse," the Philadelphia chromosome was not present, ruling out chronic myelogenous leukemia presenting in two morphologically different blastic phases. Cytogenetic and histochemical studies both at original presentation and at the time of relapse would be helpful in establishing the occurrence of a new leukemia.