Philadelphia (Ph) chromosome-positive thrombocythemia without features of chronic myeloid leukemia in peripheral blood: natural history and diagnostic differentiation from Ph-negative essential thrombocythemia.

We have evaluated the clinical symptoms, hematological features, and natural history of 3 cases and 20 reported cases described as Philadelphia chromosome-positive (Ph+) essential thrombocythemia (ET). The presence of increased small mononuclear megakaryocytes in bone marrow smears and biopsy material in patients with pronounced thrombocytosis and no evidence of chronic myeloid leukemia (CML) in peripheral blood appeared to be a diagnostic clue to the diagnosis of Ph+ (essential) thrombocythemia. As compared to cases of reactive thrombocytosis, the megakaryocytes in Ph+ thrombocythemia are smaller than normal ones and typically have hypolobulated round nuclei. This contrasts with the finding of clustered mature and enlarged megakaryocytes in Ph-negative true ET. Patients diagnosed as Ph+ ET may progress to CML and show a high tendency to myelofibrosis and blastic transformation. These observations indicate that both Ph+ ET and Ph+ thrombocythemia associated with CML can be regarded as early manifestations of the chronic stable phase of CML.