| Summary: | <strong>Purpose: </strong> Cytoreductive therapy is beneficial in patients with essential thrombocythemia at high risk of thrombosis. However its value in those lacking high-risk features remains unknown. This open-label, randomized trial compared hydroxycarbamide plus aspirin with aspirin alone in patients with ET aged 40- 59 and without high-risk factors or extreme thrombocytosis. <strong>Methods:</strong> Patients were aged 40-59 years and lacked a history of ischemia, thrombosis, embolism, hemorrhage, extreme thrombocytosis (≥1500x109 /L), or hypertension or diabetes requiring therapy. 382 patients were randomized 1:1 to hydroxycarbamide plus aspirin or aspirin alone. The composite primary endpoint was time to arterial or venous thrombosis, serious hemorrhage or death from vascular causes. Secondary endpoints were time to first arterial or venous thrombosis, first serious hemorrhage, death, incidence of transformation and patient-reported quality of life. <strong>Results:</strong> After a median follow-up of 73 months and a total follow-up of 2373 patient years there was no significant difference between the arms in the likelihood of patients reaching the primary endpoint (hazard ratio 0.98, 95% CI 0.42-2.25, P=1.0). The incidence of significant vascular events was low, at 0.93 per 100 patient-years (95% CI 0.61-1.41). There were also no differences in overall 4 survival, in the composite endpoint of transformation to myelofibrosis, acute myeloid leukaemia or myelodysplasia, in adverse events or in patient-reported quality of life. <strong>Conclusions:</strong> In ET patients aged 40-59 and lacking high-risk factors for thrombosis or extreme thrombocytosis, pre-emptive addition of hydroxycarbamide to aspirin did not reduce vascular events, myelofibrotic or leukemic transformation. Patients aged 40-59 without other clinical indications for treatment (such as previous thrombosis or haemorrhage), who have a platelet count less than 1500x109 /L, should not receive cytoreductive therapy.
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