Note: * Denotes a randomized trial; n.s., nonsignificant; n.r., not reported. ASA, Acetyl salicylic acid.

sion with either radioactive phosphorus (32P) or the alkylating agent chlorambucil. The median survival of the phlebotomy arm (13.9 years) was superior to that of either the 32P arm (11.8 years) or the chlorambucil arm (8.9 years) (p = .02).103 However, there was a markedly increased risk of thrombosis in the phlebotomy arm, particularly within the first 3 years and in patients with previous thrombosis or advanced age (p = .015). There was also a startling difference in the quoted incidence of acute leukemia between the randomization arms. After follow-up of between 13 and 15 years, the incidence of leukemia in the phlebotomy arm was only 1.5 percent, whereas it was 10 percent for 32P and 13 percent for chlorambucil (p = .0045). The incidence of lymphomas and solid organ tumors was also increased for the two myelosuppressive arms.

The results of this study need to be interpreted with caution, however. The higher thrombosis rate with phlebotomy was thought likely to be due to the uncontrolled thrombocythemia in some patients receiving treatment with phlebotomy alone. This may have been compounded by the initial target hematocrit of 0.52, which was subsequently reduced following the description of the influence of the hematocrit on blood flow and thrombosis.104'105 After the target hematocrit was reset at 0.45 the rate of thrombosis was the same in all three arms. The incidence of leukemia in the phlebotomy arm is also potentially misleading. Any patient requiring myelosuppression (90 percent of patients by 10 years included in the PVSG-01 study by Najean and Rain18) was censored from the analysis and so the AML incidence of 1.5 percent refers to a selected subgroup of patients with relatively benign disease.20

There has also been concern over an increased incidence of myelofibrosis in PV patients treated with phlebotomy alone, possibly due to the release of fibrogenic cytokines from elevated platelet and megakaryocyte numbers. In one study19 the actuarial risk of developing myelofibrosis was higher in patients treated with phlebotomy alone than in patients treated initially with 32P (some of whom later received hydroxyurea). Similar observations have been reported by another group106 but not by the PVSG.107 Thus, although phlebotomy is the initial therapy of choice for patients with PV, additional myelosuppression is necessary in most patients at some time during the course of the disease.

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