Secondary prevention of cancer: an overview.

Secondary prevention of cancer (screening) involves the use of tests to detect a cancer before the appearance of signs or symptoms. Before starting such a programme, the available evidence should be analysed to estimate the effectiveness of the proposed activities. Essential requirements are an understanding of the natural history of the particular cancer, availability of a test that can detect it, effective treatment for it, good evidence that early detection reduces the incidence and/or mortality, and that the expected benefits of screening outweigh the risks and costs. A screening programme should be limited to significant cancers and applied selectively, and should be integrated into the total health care programme. Programmes should take into account the risks, costs and expected benefits; provide quality assurance as well as facilities to follow, diagnose, and treat people with positive test results; maintain all records; and keep costs to a minimum. Ideally the effectiveness of screening should be demonstrated by randomized controlled trials showing a reduction in mortality, but this type of evidence exists for few cancers. Often an estimate of the effectiveness of screening must rest on other types of evidence, such as observations that the tests can detect the cancer before the appearance of signs or symptoms; that the tests can find a greater proportion of cancers in early stages; and that the patients with cancers detected through screening have higher survival rates after diagnosis and treatment although it must be recognized that these observations may be biased. This article discusses the available evidence on the effectiveness of screening for eight cancers, and gives estimates of the potential impact of secondary prevention for the year 2000.