Mammographic screening: case–control studies

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Abstract
Background: The case–control design can be used to evaluate the benefit of cancer screening programmes. Materials and methods: This paper outlines the main methodological features of the case–control design in this context, and indicates some potential biases. It also reviews the existing case–control literature on mammographic screening. Results: Case–control studies consistently indicate a reduction of ∼50% in breast cancer mortality associated with mammography. This result indicates greater benefit than shown in randomised trials; however, one should recognise that trials indicate effectiveness whereas case–control studies indicate efficacy. The two types of evidence are broadly compatible when one allows for screening non-compliance and contamination in the randomised trials. Conclusions: The case–control evidence supports and is consistent with the findings of randomised trials of mammography. Effectiveness estimates from trials indicate the benefit of screening to the population as a whole, and are pertinent to the public policy debate as to the value of offering screening. In contrast, case–control studies indicate benefit to actual screening participants. As such, case–control estimates of efficacy are appropriate for individual decision-making by women about their use of mammography when it is potentially available to them.