Leukemic transformation in polycythemia Vera. MPD(UK) Study Group.

The recent report by Najean and Rain for the French Polycythemia Study Group1 comparing hydroxyurea with pipobroman in the treatment of polycythemia vera (PV) makes an important contribution to the debate about the treatment of this condition. We would like to make some observations about the presentation of their results. This particularly relates to their statements about the observed actuarial risk of leukemia. The investigators chose, both in the text and the summary, to state that for both drugs "the risk of leukemia is approximately 10% at the 13th year." This statement may be misleading if it is compared to other studies without presenting confidence intervals for their observation and also appreciating that other studies often quote an overall percentage incidence of leukemia rather than actuarial risk. In their study approximately 27% and 15% of the total number of patients involved remained under observation at the 10th and 13th year, respectively. Thus, by the 13th year the number of patients remaining was relatively small and the accuracy of the actuarial risk of leukemia at this timepoint is likely to be suspect. We suggest that since the number remaining at 10 years was considerably larger, the investigators should have focused in the text and in their abstract on the actuarial risk of leukemia at that time. From their data, this is approximately 5% and 3% for hydroxyurea and pipobroman, respectively. This compares with the actuarial risk of leukemia at 10 years in the PVSG-01 study2 of approximately 16% and 18% in the 32P and chlorambucil limbs, respectively. Also in this study the estimates of risk of leukemia beyond 10 years are unstable because of the small number of patients still at risk