Advanced age and a history of thrombosis were well-established risk factors for thrombosis in essential thrombocythemia (ET) and polycythemia vera (PV); cytoreductive therapy was indicated in their presence. Recent studies have suggested leukocytosis as an additional risk factor; however, such an association would be treatment-relevant in the context of low-risk disease.
In this retrospective study, a Cox proportional hazards model was used to determine the impact of various clinical and laboratory variables, including leukocytosis, on thrombosis-free survival (TFS). Arterial-specific or venous-specific TFS curves for different leukocyte count-defined risk groups were constructed by the Kaplan-Meier method and compared using the log-rank test.
A total of 407 low-risk patients (254 with ET and 153 with PV) were considered. After a respective median follow-up of 104 months and 130 months, respectively, 47 (19%) patients with ET and 41 (27%) with PV experienced a total of 55 (41 arterial and 14 venous) and 46 (22 arterial and 24 venous) thrombotic events, respectively. Leukocytosis at the time of diagnosis, defined by a cutoff level of either 15 or 9.4 × 109/L, did not appear to be predictive of either arterial or venous thrombosis during follow-up; similar results were obtained when analysis was restricted to patients with platelet counts of <1000 × 109/L. Instead, advanced age was found to be significantly associated with arterial thrombosis in patients with PV and higher hemoglobin level with venous thrombosis in patients with ET.
In the current retrospective study, leukocytosis at diagnosis did not appear to influence the risk of thrombosis in either ET or PV. However, a prospective study is required before leukocytosis is taken into account during treatment decisions in these disorders. Cancer 2009. © 2009 American Cancer Society.