Excess mortality from breast cancer in relation to mammography screening in northern Sweden

Previous randomised studies of mammography screening have shown a significant effect on breast cancer mortality, particularly in women aged 50-69 at randomisation. Breast cancer mortality has traditionally been studied by judgments on causes of death, either from cause of death registers or from medical records. In this study an alternative method was used, estimating the excess mortality associated with breast cancer.

In 1990 two counties of northern Sweden started population based mammography screening of women aged 40-74. The unscreened population in the two other counties of the same region were selected as controls.

Excess mortality associated with breast cancer was lower in the screened population, and was discernible three to four years after the start of screening. The relative risk estimate, based on the cumulative excess number of deaths from breast cancer during 1990-95 in the screened versus the control population aged 40-74 (at diagnosis of breast cancer), was 0.72 (95% confidence interval (CI) 0.53 to 0.99). For women aged 50-69 it was 0.67 (95% CI 0.46 to 0.99). In the 50-69 age group the estimated excess number of deaths from breast cancer during 1995 was 17.0 per 100,000 women (95% CI 5.0 to 29.0) in the screened counties and 51.1 per 100,000 (95% CI 30.2 to 71.9) in the unscreened counties.

Population based routine screening has substantial effects on breast cancer mortality in women aged 50-69. Estimation of excess mortality can be used in future studies to evaluate the effects of mammography screening on breast cancer mortality.