Absence of sustained breast cancer incidence inflation in a national mammography screening programme

Objective To investigate the impact of population mammography screening on breast cancer incidence trends in New Zealand. Methods Trends in age-specific rates of invasive breast cancer incidence (1994–2014) were assessed in relation to screening in women aged 50–64 from 1999 and 45–69 following the programme age extension in mid-2004. Results Breast cancer incidence increased significantly by 18% in women aged 50–64 compared with 1994–98 (p<0.0001), coinciding with the 1999 introduction of mammography screening, and remained elevated for four years, before declining to pre-screening levels. Increases over 1994–99 incidence occurred in the 45–49 (21%) and 65–69 (19%) age groups following the 2004 age extension (p<0.0001). Following establishment of screening (2006–10), elevated incidence in the screening target age groups was compensated for by lower incidence in the post-screening ⩾70 age groups than in 1994–98. Incidence in women aged ⩾45 was not significantly higher (+5%) after 2006 than in 1994–98. The cumulated risk of breast cancer in women aged 45–84 for 1994–98 was 10.7% compared with 10.8% in 2006–10. Conclusions Increases in breast cancer incidence following introduction of mammography screening in women aged 50–64 did not persist. Incidence inflation also occurred after introduction of screening for age groups 45–49 and 65–69. The cumulated incidence for women aged 45–84 over 2006–10 after screening was well established, compared with 1994–98 prior to screening, shows no increase in diagnosis. Over-diagnosis is not inevitable in population mammography screening programmes.