Service screening with mammography in Northern Sweden: effects on breast cancer mortality - an update.

Objectives: To study the effectiveness of service screening with mammography in Northern Sweden.

Setting: Two counties which invited women aged 40-74 years to service screening with mammography were compared with two counties where service screening started 5-7 years later. There were 109,000 and 77,000 women in the study and control counties, respectively.

Methods: Cohorts in the study group were defined to include only breast cancer cases diagnosed after their first invitation to screening. Two outcome measures for breast cancer mortality were used; excess mortality and underlying cause of death (UCD). Detection mode was used to estimate the efficacy of screening for those women who actually attended screening. The cohorts were followed for 11 years.

Results: The relative rate (RR) of breast cancer death as excess mortality and UCD for women aged 40-74 years invited to screening, compared with women not yet invited, was 0.70 (95% confidence interval [CI] 0.56-0.87) and 0.74 (95% CI 0.62-0.88), respectively. The largest effect was seen in women aged 40-49 years (RR = 0.64 and RR = 0.62 for excess mortality and UCD, respectively). RR in age 40-74 years for women actually screened was 0.65 (95% CI 0.51-0.84) and 0.70 (95% CI 0.57-0.86) for excess mortality and UCD, respectively. The number of women needed to screen to save one life was 912 after 11 years of follow-up.

Conclusions: This study confirms previous findings in the earlier follow-up and indicates a long-term reduction of breast cancer mortality by 26-30%. The efficacy among those who actually attended screening was about 5% larger.