Philadelphia chromosome positive essential thrombocythemia evolving into lymphoid blast crisis.

The Philadelphia chromosome with the classic translocation t(9;22) was detected in all metaphases of a patient with essential thrombocythemia who presented without hematologic signs of chronic myeloid leukemia (CML). After 12 months of treatment with hydroxyurea followed by 6 months of busulfan, the disease transformed into lymphoid blast crisis with the immunophenotype of pre-pre-B cells frequently seen in lymphoid blast crisis of CML. This progression was not associated with karyotypic evolution because lymphoblasts contained only the Philadelphia chromosome. Further clinical course in this patient confirmed the development of a Philadelphia chromosome positive stem cell with predominantly megakaryocytic committment through partially megakaryocytic/lymphoid differentiation to purely lymphoid committment, a previously unreported transformation.