Improved stage-specific survival in screen-detected breast cancer in Denmark: a cohort study

BACKGROUND: This study examined whether breast cancer survival improvements with screening are explained solely by detection at early stages and whether mortality can be predicted using stage and diagnosis date alone. METHODS: We compared stage-specific net survival between never-screened, symptomatic ever-screened (past attenders and interval cancers), and screen-detected breast cancer cases in Denmark using individual-level electronic health records from January 1, 2010 to December 31, 2022. Lifetables were generated from women without breast cancer, separately for never-screened and ever-screened women. Age-specific all-cause mortality rates were used to calculate excess mortality in women with breast cancer, which was then transformed into net survival. RESULTS: Of 817 128 women, 32 827 had breast cancer, with 8% presenting as stage III or IV. Survival differences between symptomatic and screen-detected cases were minimal for stages I-III but reached 40% at stage IV, with 5-year net survival for stage IV screen-detected women (74.7%) resembling stage IIIc symptomatic survival in never-screened women (72.6%). Survival from stage IV breast cancer was strongly associated with treatment, with median survival (years) of 4.4-6.0 with surgery, 1.6-2.2 with nonsurgical treatment, and 0.03-0.13 with no treatment; 67% of screen-detected patients received surgery (compared with 23% of never-screened and 27% of symptomatic ever-screened). CONCLUSIONS: Greater survival in screen-detected stage IV cases suggests that breast cancer screening may not have come too late and deserves to be investigated further. Predicting breast cancer mortality using stage at diagnosis and stage-specific survival (without differentiating by route to diagnosis) will underestimate the impact of breast screening on mortality.