Ph-positive chronic myeloid leukemia mimicking essential thrombocythemia and terminating into megakaryoblastic blast crisis: report of two cases with molecular studies.

Two patients with chronic myeloid leukemia (CML) presenting with the hematologic features of essential thrombocythemia (ET) are reported. At diagnosis they showed extremely high platelet counts (4985 and 2800 x 10(9)/l, respectively) and moderate leukocytosis (21 and 17 x 10(9)/l, respectively). In both cases, in addition to the Philadelphia chromosome (Ph), a rearrangement within the major breakpoint cluster region on chromosome 22 was demonstrated, with the breakpoint in the 3' extreme. In patient 1 the disease initially responded to radioactive phosphorus and hydroxyurea, but during the evolutive course a progressive increase in the white blood cell counts was noted, reaching values typical of the chronic phase of CML, and the patient eventually died from blast crisis 45 months after diagnosis. In patient 2, although good control of the platelet counts was achieved with hydroxyurea, the disease also evolved into a blast crisis four months after diagnosis. In both cases monoclonal antibodies and electron microscopy studies demonstrated the megakaryocytic nature of the blast cells. The above features are not consistent with the present and similar cases being Ph-positive ET. Instead, they should be regarded as a special form of CML characterized by a marked protagonism of the megakaryocytic component.