Transformation of Chronic Myeloid Leukemia to Acute Biphenotypic Leukemia

Chronic myeloid leukemia (CML) is a myeloproliferative disorder with clonal proliferation of all myeloid cell lines. The disease typically manifests in three phases: chronic course followed by an accelerated phase and finally a terminal blast crisis. A blast crisis is defined as the presence of > 20% blasts in the peripheral blood or bone marrow. The blasts could be characterized as either myeloid (60-80% of cases) resulting in acute myeloblastic leukemia or lymphoid (20-30% of cases) resulting in acute lymphoblastic leukemia. In rare instances, a blast crisis could present with biphenotypic expression for both myeloid and lymphoid blasts. In such cases, about 6-10% of the time, the course of the disease is more aggressive and renders a poorer prognosis especially if there is evidence of extramedullary involvement. We present a case of a 41-year-old woman with history of CML who presented with acute biphenotypic blast crisis with extramedullary involvement in the context of aggressive chemotherapy. Literature review reveals < 10 reported cases of CML with biphasic transformation and only three cases of CML with acute leukemia on presentation. Most cases described are in pediatric patients with varied presentations and none involving extramedullary sites. Mortality rates in such cases are near 100% despite aggressive chemotherapy.