ArticlesLong-term effects of mammography screening: updated overview of the Swedish randomised trials
Introduction
Service-screening for breast cancer occurs in several countries with the aim to decrease breast cancer mortality. The scientific basis for these programmes are the randomised screening trials. There are seven such studies, four from Sweden. The Swedish trials have a similar design: they were all population-based and compared invitation to breast screening with mammography alone versus no invitation. These Swedish trials differed from the other trials (the Health Insurance Plan of Greater New York, the Edinburgh trial, and the Canadian National Breast Screening Study), which all evaluated mammography combined with breast self-examination, clinical breast examination, or both. Moreover, the New York and the Canadian trials were not population-based.
The Swedish Cancer Society initiated an overview of the Swedish trials in the late 1980s. The objective was to validate the results from the individual trials through a method that was common to all trials, including a blind review of all deaths among breast cancer cases by an independent endpoint committee. Another objective was to increase the statistical power. The first results of the overview were published in 19931 and an-update focusing on the age group 40-49 in 1997.2 Concerns raised about the validity of the results from the trials.3 include inappropriate exclusions, poor randomisation, and the excess total mortality in women invited to screening.
Our aim here was to extend the follow-up and to analyse the age-specific and trial-specific effects on breast cancer mortality, to describe the randomisation procedures in more detail, and to assess the quality of the cluster randomisation used in Östergötland. The Kopparberg part of the Two-County (WE) trial was not available for analysis but the continuation of the Malmö trial (MMST II) was added.
Section snippets
Methods
Details of the Swedish mammography screening trials have been published (table 1), and are summarised below.
Malmö trial
The Malmö Mammographic Screening Trial (MMST) included women in the city of Malmö from October, 1976. In the first part (MMST I) women born between 1908 and 1932 were randomised with individual stratification by year of birth. Women were invited to screen-film mammography alone, in the first two rounds with two views (craniocaudal and oblique) and in subsequent rounds with either two views or the oblique view alone depending on the parenchymal pattern. A single oblique view was used for women
Number of women randomised, trial time, and follow-up time
Our analysis was based on the follow-up of 247 010 women, 129 750 of whom were invited to mammography screening and 117 260 of whom were controls. 4001 women below the age of 40 and 14 959 women from Östergötland aged 75 and above were excluded. Age distribution by trial is in table 2
Median trial time and range are in table 3. Trial time was defined as time from date of randomisation until the control groups completed the first round of screening. In trials in which the control groups were not
Discussion
Our aim was to elucidate some issues that have been raised in recent reviews of the Swedish trials. In addition, we wanted to assess the long-term effects on mortality, including age-specific and trial-specific effects. Our latest overview, which is unbiased and unconfounded for study design, confirms and extends previous results. Our main observation was that the benefit of screening was maintained several years after the trials had been closed. In general, the benefit in absolute terms
References (15)
- et al.
Breast cancer screening with mammography: overview of Swedish randomised studies.
Lancet
(1993) - et al.
Is screening for breast cancer with mammography justifiable?
Lancet
(2000) - et al.
Reduction in mortality from breast cancer after mass screening with mammography: randomised trial from the Breast Cancer Screening Working Group of the Swedish National Board of Health and Welfare.
Lancet
(1985) - et al.
14 years of follow-up from the Edinburgh randomised trial of breast-cancer screening.
Lancet
(1999) - et al.
The Swedish Two-County trial twenty years later: updated mortality results and new insights from long-term follow-up.
Radiol Clin North Am
(2000) - et al.
Updated overview of the Swedish randomised trials on breast cancer screening with mammography: age group 40-49 at randomisation.
J Natl Cancer Inst Mono
(1997) - et al.
Mammographic screening and mortality from breast cancer: the Malmö Mammographic Screening Trial.
BMJ
(1988)