The Swedish Two-County Trial of mammographic screening: cluster randomisation and end point evaluation

• Published in: Annals of oncology Vol. 14; no. 8; pp. 1196 - 1198
• Main Authors: Duffy, S. W., Tabar, L., Vitak, B., Yen, M. F., Warwick, J., Smith, R. A., Chen, H. H.
• Format: Journal Article
• Language: English
• Published: Oxford Oxford University Press 01.08.2003
• Subjects: Adult; Aged; Attitude to Health; Biological and medical sciences; Breast Neoplasms - mortality; Breast Neoplasms - prevention & control; Breast screening; Cluster Analysis; Female; Gynecology. Andrology. Obstetrics; Humans; Key words: breast screening; Mammary gland diseases; Mammography; Mammography - methods; Mass Screening - methods; Medical sciences; Middle Aged; NATURAL SCIENCES; NATURVETENSKAP; Patient Participation; Prevalence; Randomised trial; Reference Values; Risk Assessment; Sensitivity and Specificity; Survival Analysis; Sweden - epidemiology; Tumors; Sweden; Europe; Human; breast screening; Mammary gland diseases; Randomization; randomised trial; Female; Mammography; Mammary gland; Malignant tumor; Medical screening; Epidemiology
• ISSN: 0923-7534; 1569-8041; 1569-8041
• DOI: 10.1093/annonc/mdg322

Background: The Swedish Two-County Trial has been criticised on the grounds of the cluster randomisation and alleged bias in classification of cause of death. Patients and methods: In the Two-County Trial, 77 080 women were randomised to regular invitation to screening (active study population, ASP) and 55 985 to no invitation (passive study population, PSP), in 45 geographical clusters. After ∼7 years, the PSP was invited to screening and the trial closed. We analysed data using hierarchical statistical models to take account of cluster randomisation, and performed a conservative analysis assuming a systematic difference between ASP and PSP in baseline breast cancer mortality in one of the counties. We also analysed deaths from causes other than breast cancer and from all causes among breast cancer cases diagnosed in the ASP and PSP. Results: Taking account of the cluster randomisation there was a significant 30% reduction in breast cancer mortality in the ASP. Conservatively, assuming a systematic difference between ASP and PSP clusters in baseline breast cancer mortality, there was a significant 27% reduction in mortality in the ASP. Ignoring classification of cause of death, there was a significant 13% reduction in all-cause mortality in breast cancer cases in the ASP. Conclusions: Breast cancer mortality is a valid end point and mammographic screening does indeed reduce mortality from breast cancer. The criticisms of the Swedish Two-County Trial are unfounded.