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.