Improved breast cancer survival following introduction of an organized mammography screening program among both screened and unscreened women: a population-based cohort study

Abstract

Introduction: Mammography screening reduces breast cancer mortality through earlier diagnosis but may convey further benefit if screening is associated with optimized treatment through multidisciplinary medical care. In Norway, a national mammography screening program was introduced among women aged 50 to 69 years during 1995/6 to 2004. Also during this time, multidisciplinary breast cancer care units were implemented.

Methods: We constructed three cohorts of breast cancer patients: 1) the pre-program group comprising women diagnosed and treated before mammography screening began in their county of residence, 2) the post-program group comprising women diagnosed and treated through multidisciplinary breast cancer care units in their county but before they had been invited to mammography screening; and 3) the screening group comprising women diagnosed and treated after invitation to screening. We calculated Kaplan-Meier plots and multivariable Cox proportional hazard models.

Results: We studied 41,833 women with breast cancer. The nine-year breast cancer-specific survival rate was 0.66 (95%CI: 0.65 to 0.67) in the pre-program group; 0.72 (95%CI: 0.70 to 0.74) in the post-program group; and 0.84 (95%CI: 0.80 to 0.88) in the screening group. In multivariable analyses, the risk of death from breast cancer was 14% lower in the post-program group than in the pre-program group (hazard ratio 0.86; (95%CI: 0.78 to 0.95, P = 0.003)).

Conclusions: After nine years follow-up, at least 33% of the improved survival is attributable to improved breast cancer management through multidisciplinary medical care.

Figure 1

The staggered implementation of the Norwegian breast cancer screening program in the different groups of counties during the study period. The three study groups are shown in different colours. A = The counties of Rogaland, Oslo, Hordaland, and Akershus; B = Telemark, Agder, Troms, and Finnmark; C = Østfold, Nordland, Buskerud, and Trøndelag; D = Oppland and Møre og Romsdal; E = Sogn og Fjordane and Hedmark; F = Vestfold.

Figure 2

Survival curves for the different groups.

Figure 3

Five-year survival of breast cancer by two-year increments throughout the study period for all women except those invited to mammography screening. The curve is presented with 95% confidence interval.