The two main goals of modelling cancer screening are data analysis and evaluation. In data analysis, analytical-numerical statistical models are used to test hypotheses about preclinical disease, the screening test, and the association between early detection and risk of dying from the cancer. Evaluation in cancer screening is supported by model-based prediction of screening effects and cost-effectiveness. Simulation models are suitable for these tasks, and can also be used to identify efficient age-ranges and intervals between screening tests. Striking differences exist between screening models for cervical cancer and breast cancer, which are the two cancer types for which screening is common practice. The two main problems in cervical cancer screening are the proportion of progressive and regressive among screen-detected lesions, and the impact of screening on incidence and mortality. In breast cancer, regression is not (yet) a big issue, and the relationship between screening and mortality reduction has been demonstrated in randomized controlled trials (at least for women older than 50 years). The weakest link in current breast cancer models is the association between earliness of detection and improvement in prognosis. The modelling outcomes and their usefulness are decisively influenced by the data sets that were used in quantifying the model, and the subclassifications of the data that were considered. New or pending modelling issues include HPV-based screening in cervical cancer, screening models for colorectal cancer, the use of surrogate outcome measures and model-based meta-analysis of screening trials.
Summary Breast cancer screening is generally accepted as an effective means of reducing breast cancer mortality in post-menopausal women. In this analysis the impact of nationwide screening on clinical medicine and the effects for the women involved are quantified. Effect estimates are based on results from screening trials in Utrecht (DOM-project) and Nijmegen, and on bi-annual screening of women aged 50-70. The consequences for health care are based on generally accepted assessment and treatment policies. The number of assessment procedures for non-palpable lesions will increase by 12% per year in the build-up period, and will remain slightly higher. The total number of biopsies in a real population is expected to decrease. Screening will lead to a shift in primary treatment modalities, as 15% of mastectomies will be replaced by breast conserving therapy. The temporary increase in the demand for primary treatment in the first years will be followed by a decrease in the demand for treating women with advanced disease. Favourable effects outweigh the inevitable unfavourable effects, with high quality screening and an appropriate invitation system. Breast cancer screening can also be recommended after considering other consequences than mortality reduction.Several trials have shown that mammographic screening of post-menopausal women reduces breast cancer mortality (Shapiro et al., 1982;Verbeek et al., 1984;Collette et al., 1984;Tabar et al., 1985;UK Trial 1988;Andersson et al., 1988). The introduction of a national programme in the United Kingdom, offering tri-annual screening to women aged 50-65, would result in a mortality reduction of 8% (Knox, 1988). In the Netherlands with bi-annual screening of women aged 50-70, this figure would be 12% (van der Maas et al., 1989).Although mortality reduction is the fundamental effect (Day et al., 1989), there is much debate about other desirable and undesirable consequences of breast cancer screening (Warren, 1988;Skrabanek, 1988). However, publications which quantify these consequences are lacking. Starting to screen will result in a temporary increase in the number of women with newly diagnosed breast cancer. Detecting these cancers at an earlier stage will affect the type of assessment and treatment. Mass screening will also generate referrals of women, who appear to have no breast cancer (false positives) (Forrest, 1986 Assessment and excision biopsyIn assessing breast abnormalities, at least three successive steps were distinguished: physical examination, clinical mammography and excision biopsy (with possible specimen radiography). Additional possibilities which were taken into account for palpable lesions only were fine needle aspiration (cytology) and ultrasound.Estimates of the number and type of these procedures used in the situation without screening were based on the following sources: general practitioners' registry (Trouw, 1986); the radioactivity and radiation application division of the Ministry of Welfare, Health and Cultural Affairs; the Central Office ...
The estimation of prevalence and incidence of parasitic infections is considered. As the detectability of such infections is not 100% and may furthermore depend on their intensity, statistical methods are often required to arrive at meaningful results. It appears to be essential to distinguish between parasites that multiply within the (human) host and those that do not. An overview of some models discussed in the literature is presented. These models can indeed be used in assessing detectability of infection, and they indicate that observations may lead to considerable misinterpretation of 'true' prevalences and incidences.
Objective-To assess the relative protection against death from cervical cancer after two or more negative smear test results and compare it with the protection against invasive cancer estimated by an International Agency for Research on Cancer (IARC) working group in an analysis of data from 10 large screening programmes.Design-Comparison of risk of death from cervical cancer after two or more negative smear results with the risk in unscreened women by using a model constructed with data from the British Columbia screening programme.Main outcome measures -Mortality from and incidence of invasive cancer.Results-In women with two negative smear results estimates of protection against cervical cancer were about 50% higher when lethal invasive cancer was used as the criterion rather than all invasive cancer. This difference was due to these women being more likely to attend for further tests at which invasive cancer could be detected: screen detected cancer has a better prognosis than clinically diagnosed cancer. Screening intervals could be longer than three years: screening women aged 35-64 every five years was predicted to result in a 90% reduction in mortality from cervical cancer.Conclusion-Because protection from mortality is higher than protection from disease and because of the high costs and negative side effects of frequent screening, screening intervals should be longer than three years. IntroductionThe International Agency for Research on Cancer (IARC) working group on evaluation of cervical cancer screening programmes has analysed data from large screening programmes in Europe and North America.' Protection against cervical cancer after negative smear test results was measured by the risk ofinvasive cancer. The relative protection-that is, the ratio of the risks in unscreened and screened women-decreases with
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