The NHS Breast Screening Programme (NHSBSP) routinely invites women aged 50-70 years to come for triennial screening. Because of uncertainty about the effects of screening outside this age range, an England-wide cluster-randomised trial is under way to assess reliably the risks and benefits of additional invitations for screening before age 50 and, separately, after age 70. Random allocation of small clusters of participants is used to determine (in a 50:50 ratio) which women are offered one additional screening invitation before age 50 and which are not, and which women are offered additional screening after age 70 and which are not. The principal outcome for screening before age 50 and, separately, after age 70 will be breast cancer mortality, eventually subdivided by 5-year time periods (0-4, 5-9, 10-14 years, etc) since random allocation. Subsidiary analyses will assess effects on other outcomes. The main results are expected in the mid 2020s.

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