Hormone replacement therapy and incidence of central nervous system tumours in the Million Women Study

We examined the relation between the use of hormone replacement therapy (HRT) and the incidence of central nervous system (CNS) tumours in a large prospective study of 1,147,894 postmenopausal women. Women were aged 56.6 years on average at entry, and HRT use was recorded at recruitment and updated, where possible, about 3 years later. During a mean follow‐up of 5.3 years per woman, 1,266 CNS tumours were diagnosed, including 557 gliomas, 311 meningiomas and 117 acoustic neuromas. Compared with never users of HRT, the relative risks (RRs) for all incident CNS tumours, gliomas, meningiomas and acoustic neuromas in current users of HRT were 1.20 (95% CI: 1.05–1.36), 1.09 (95% CI: 0.89–1.32), 1.34 (95% CI: 1.03–1.75) and 1.58 (95% CI: 1.02–2.45), respectively, and there was no significant difference in the relative risks by tumour type (heterogeneity p = 0.2). In past users of HRT the relative risk was 1.07 (95% CI: 0.93–1.24) for all CNS tumours. Among current users of HRT, there was significant heterogeneity by the type of HRT with the users of oestrogen‐only HRT at higher risk of all CNS tumours than users of oestrogen–progestagen HRT (RR = 1.42, 95% CI: 1.21–1.67 versus RR = 0.97, 95% CI: 0.82–1.16) (heterogeneity p < 0.001). Among current users of oestrogen‐only and oestrogen–progestagen HRT, there was no significant heterogeneity by duration of use, hormonal constituent or mode of administration of HRT.