Department of Epidemiology and Biostatistics, University Medical Centre Nijmegen, Nijmegen, The Netherlands
Received 5 July 2002; accepted 3 April 2003
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Abstract |
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Key words: breast cancer service screening, evaluation
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Introduction |
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The gradual implementation of the national screening network took more than 10 years to complete [2]. In Table 1 the early outcomes aggregated for the period 19901999 are presented for women screened initially, and for subsequently screened women [2]. For the subsequent screening category, i.e. a previous screening examination <2.5 years before, the results are 7.4 referrals for clinical assessment per 1000 women screened, 4.7 biopsies and 3.6 breast cancers detected. The stage distribution of detected cancers is favourable with 78% either in situ or T1 tumours of at most 20 mm, and 23% axillary lymph node positivity. Currently, the attendance rate is 80%, and 800 000 women are examined annually.
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Fifty per cent would have been cured; they would also have survived without a screening programme. A further 16% will die from breast cancer despite early detection, as it was not detected early enough. Another 5% would have died from other causes before clinical manifestation; that is overdiagnosis and/or death in the lead time period [6]. Finally, the remaining 28% will benefit: they will not die from breast cancer as a positive result of screening. In The Netherlands, these 28% will comprise 700 (detected through subsequent and initial screening) breast cancer cases every year who will not die from breast cancer as a positive result of screening.
The results of the randomised controlled trials indicate that a reduction in breast cancer mortality related to screening will take 710 years to emerge. Given the phased implementation of the Dutch national screening programme, it may still be too early to evaluate the impact of screening on mortality. The NETB uses the MISCAN (Microsimulation Screening Analysis) model to predict age-specific breast cancer mortality rates (in the presence and absence of a screening programme) and annually compares these rates to the observed breast cancer mortality rates in The Netherlands. Up to 1996, there is no significant difference between observed mortality and expected mortality in the absence of screening [7]. In 1999, for the second time in succession for women aged 5069 years, the breast cancer mortality rates as observed by The Netherlands Central Bureau of Statistics were statistically significantly lower than would have been expected in a situation without a national screening programme.
Although useful and important, ultimate proof that a population service-based screening programme is effective can never be based solely on the analysis of regional or national trends in breast cancer mortality. After all, declining breast cancer mortality rates could also be ascribed (wholly or partially) to an overall improvement in breast cancer prognosis due to earlier diagnosis outside screening and detection of possibly less aggressive tumours. Additional explanations include treatment advances, stringency of clinical management and changes in risk factors over time.
An extensive mortality evaluation needs careful planning, even more since this effort should become a routine and fundamental part of the evaluation of the Dutch breast cancer screening programme [8]. The only way to achieve a reliable assessment of the effect of screening on mortality is on the basis of longitudinal individual data, directly linking a womans screening history to her cause of death. This requires a linkage between the invitation and screening registries of the regional screening organisations, the regional cancer registries and the cause of death registry at Statistics Netherlands. At present, the strict Dutch privacy legislation does not allow this linkage for unscreened women since their consent to use data on their screening history is not available. A pilot linkage is currently underway for one screening region, which may identify additional bottlenecks in the procedure.
If we find a way to continuously carry out the above linkage, additional information will still be needed regarding possible confounders and treatment to perform analyses at a more detailed level. Collecting this information periodically from a sample of the target population will enable us to perform a well-designed case-referent study at a reasonable cost. A large-scale pilot case-referent analysis of the Nijmegen screening programme has shown that such a study should pay careful attention to the definition of the exposure to screening in relation to the relevant time window for screening. These analyses should be carried out according to the actual age at which women were screened. Furthermore, histopathological and mammographical parameters, as well as clinical management issues, are of equipoise pivotal importance as determinants of breast cancer mortality reduction.
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Acknowledgements |
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Footnotes |
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References |
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