Polycythemia vera (PCV) is a well-recognized clinical and pathological entity characterized by trilineage hyperplasia of the bone marrow with increased numbers of circulating ery-throcytes, granulocytes, and platelets.140 Splenomegaly is almost always present, but hepatomegaly is less common, although both organs display EMH. PCV is an acquired monoclonal disorder of a pluripotent stem cell resulting in expansion of committed stem cell pools, most prominently the erythroid series.140,141 Erythroid progenitors proliferate independently, without requiring extrinsic erythropoietin.142 Cellular maturation is not significantly impaired and the mature peripheral blood cells function normally. The essential feature in the diagnosis of PCV is an (erythropoietin-independent) elevation of the red cell mass without an explanation for secondary erythro-cytosis.143 Bone marrow findings in PCV are not required for diagnosis and are highly variable. More than 90 percent of those with PCV exhibit hypercellular marrows and most have a moderate increase in bone marrow megakaryocytes.144 Reticulin content is usually normal or slightly increased. Only 5 percent of patients have stainable marrow iron stores, consistent with the iron depletion that typically characterizes this disease.
Patients with PCV may enter a so-called stable phase, during which normal peripheral blood counts are maintained without any specific therapy. During the stable phase, the bone marrow is gradually replaced by progressive fibrosis, and in some cases, a decrease in the proliferative capacity of the residual marrow occurs. Progressive splenomegaly, along with the appearance of anemia, and a leukoerythroblastic blood picture, including the presence of tear-drop red cell forms signify the evolution of PCV into the late (or spent) phase.145 At this time the bone marrow biopsy will reveal extensive myelofibrosis. The spent phase, which occurs in approximately 10 percent of patients with PCV, develops after mean elapsed time of 7.5 years (range 39 to 144 months) after diagnosis.145 Occasionally, patients may initially present for evaluation during this spent phase and may be diagnosed erroneously as agnogenic myeloid metaplasia unless a history of previous increase in the hematocrit or chronic iron deficiency anemia is obtained. Although prior exposure to therapeutic radiation may be a predisposing factor for post-polycythemic myeloid metaplasia, untreated patients have also developed this complication.
Transition to acute leukemia occurs in approximately 10 percent of patients with PCV.46 Most patients who transform to acute leukemia have received either chlorambucil or P32 ther-apy.146,147 However, there have been documented cases of patients treated with a phlebotomy alone who developed AML.146,147 The incidence of leukemia in those patients with post-polycythemic myeloid metaplasia was higher than in those without the spent phase, especially when myelosuppressive therapy had been used. As in many cases of post-therapy AML, myelodysplasia may also precede the transformation to acute leukemia147,148 especially in patients who have received prior alkylating agent, P32, or even hydroxyurea therapy.148 Rare cases of transformation to non-Hodgkin's lymphoma149 and acute lymphoblastic leukemia have been described,150,151 although more commonly patients with AML following PCV have blasts that bear biphenotypic markers, consistent with derivation from a pluripotent stem cell.151
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