Journal of Medical Screening > Volume 16, Number 2 > Pp. 73-80

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Original Articles

Differences in endpoints between the Swedish W-E (two county) trial of mammographic screening and the Swedish overview: methodological consequences

L Holmberg, Professor of Cancer Epidemiology  , King's College London, Medical School, Division of Cancer Studies, London, UK

S W Duffy, Professor of Breast Cancer Screening , Cancer Research UK Centre for Epidemiology, Mathematics and Statistics, Wolfson Institute of Preventive Medicine, London, UK

A M F Yen , Cancer Research UK Centre for Epidemiology, Mathematics and Statistics, Wolfson Institute of Preventive Medicine, London, UK

L Tabár, Professor of Radiology , University of Uppsala, School of Medicine, Department of Mammography, Falun Central Hospital, Falun, Sweden

B Vitak, Consultant Radiologist , Division of Radiological Sciences, Department of Medical and Health Sciences, Linköping University, Linköping, Sweden

L Nyström, Associate Professor of Epidemiology , Department of Public Health and Clinical Medicine, Umeå Universtiy, Umeå, Sweden

J Frisell, Professor of Surgery , Department of Molecular Medicine and Surgery, Unit of Breast Surgery, Karolinska Institute, Solna, Sweden

Correspondence to: Lars Holmberg, Research Oncology, 3rd floor Bermondsey Wing, Guy's Hospital, Divison of Cancer Studies, Guy's Campus, King's College London, SE1 9RT London, UK; Lars.holmberg{at}kcl.ac.uk

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Objectives To characterize and quantify the differences in the number of cases and breast cancer deaths in the Swedish W-E Trial compared with the Swedish Overview Committee (OVC) summaries and to study methodological issues related to trials in secondary prevention.

Setting The study population of the W-E Trial of mammography screening was included in the first (W and E county) and the second (E-county) OVC summary of all Swedish randomized mammography screening trials. The OVC and the W-E Trial used different criteria for case definition and causes of death determination.

Method A Review Committee compared the original data files from W and E county and the first and second OVC. The reason for a discrepancy was determined individually for all non-concordant cases or breast cancer deaths.

Results Of the 2615 cases included by the W-E Trial or the OVC, there were 478 (18%) disagreements. Of the disagreements 82% were due to inclusion/exclusion criteria, and 18% to disagreement with respect to cause of death or vital status at ascertainment. For E-County, the OVC inclusion rules and register based determination of cause of death (second OVC) rather than individual case review (W-E Trial and 1st OVC) resulted in a reduction of the estimate of the effect of screening, but for W-County the difference between the original trial and the OVC was modest.

Conclusions The conclusion that invitation to mammography screening reduces breast cancer mortality remains robust. Disagreements were mainly due to study design issues, while disagreements about cause of death were a minority. When secondary research does not adhere to the protocols of the primary research projects, the consequences of such design differences should be investigated and reported. Register linkage of trials can add follow-up information. The precision of trials with modest size is enhanced by individual monitoring of case status and outcome status such as determination of cause of death.

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