Background: Hormonal and reproductive history has been associated with risk of some hematologic malignancies, but their role in myeloproliferative neoplasms (MPN) is largely unknown. Methods: Using a population-based cohort study, we evaluated the association of these factors with risk of MPN overall, and for essential thrombocythemia (ET) and polycythemia vera (PV) specifically. Incident MPN cases from 1993 to 2004 were identified via linkage to Medicare. RR and 95% confidence intervals (CI) were estimated utilizing Cox proportional hazard regression. Results: After >250,000 person-years of follow-up, 257 cases of MPN were identified (172 ET, 64 PV). Ever use of hormone therapy (HT) was associated with an increased risk of ET (RR = 1.63; 95% CI, 1.19-2.23) but a decreased risk of PV (RR = 0.58; 95% CI, 0.34-0.98). There were no statistically significant associations of oral contraceptives or reproductive factors with MPN risk overall, or by MPN subtype. Bilateral oophorectomy was associated with increased risk of ET (RR = 1.58; 95% CI, 1.11-2.25) and decreased risk of PV (RR = 0.32; 95% CI, 0.12-0.88). There was no association of ovulatory years with ET risk; however, there was increased risk of PV (RR = 1.68 for >36.8 compared with ≤27.6 years; P trend = 0.045). Adjustment for potential confounding factors did not alter these associations. Conclusions: HT use and bilateral oophorectomy had opposite associations for ET and PV. Except for ovulatory years and PV risk, reproductive history did not appear to play a role in the etiology of MPN. Impact: This study suggests different mechanistic impacts of estrogen, and perhaps distinct etiologies, for the two major MPN subtypes.
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