Invitation to mammographic screening results in a highly significant decrease in breast cancer-specific mortality. Evaluation of the full impact of screening, in particular estimates of absolute benefit and number needed to screen, requires follow-up times exceeding 20 years because the observed number of breast cancer deaths prevented increases with increasing time of follow-up.

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The Swedish Two-County Trial is a randomized controlled study of invitation to breast cancer screening. It was initiated in late 1977. The follow-up to the end of 1998 provides results at approximately the twentieth anniversary of the trial. A significant decrease in breast cancer death among women invited to screening was published 7-8 years after randomization and at 20-year follow up there is a significant 32% reduction in mortality associated with invitation to screening. The advent of screen-film mammographic screening with the ability to detect potentially fatal tumors at an early stage provides an opportunity to study the natural history of breast cancer at an earlier phase in its development than was possible in the past. Our findings show that breast cancer is not a systemic disease at its inception, but is a progressive disease and its development can be arrested by screening. Detection of < 15 mm and lymph node negative invasive tumors will save lives and confer an opportunity for less radical treatment.Mammography is clearly a very useful tool, not only for early detection of cancers but also for successful discrimination between the highly fatal and nonfatal cancers. The four mammographic prognostic features will be presented.

ObjectivesTo estimate the absolute numbers of breast cancer deaths prevented and the absolute numbers of tumours overdiagnosed in mammographic screening for breast cancer at ages 50–69 years.SettingThe Swedish Two-County randomized trial of mammographic screening for breast cancer, and the UK Breast Screening Programme in England, ages 50–69 years.MethodsWe estimated the absolute numbers of deaths avoided and additional cases diagnosed in the study group (active study population) of the Swedish Two-County Trial, by comparison with the control group (passive study population). We estimated the same quantities for the mortality and incidence rates in England (1974–2004 and 1974–2003, respectively). We used Poisson regression for statistical inference.ResultsA substantial and significant reduction in breast cancer mortality was associated with screening in both the Two-County Trial (P < 0.001) and the screening programme in England (P < 0.001). The absolute benefits were estimated as 8.8 and 5.7 breast cancer deaths prevented per 1000 women screened for 20 years starting at age 50 from the Two-County Trial and screening programme in England, respectively. The corresponding estimated numbers of cases overdiagnosed per 1000 women screened for 20 years were, respectively, 4.3 and 2.3 per 1000.ConclusionsThe benefit of mammographic screening in terms of lives saved is greater in absolute terms than the harm in terms of overdiagnosis. Between 2 and 2.5 lives are saved for every overdiagnosed case.

In the evaluation of uncontrolled service screening programmes for cancer, the case-control design is sometimes used, in which people who die from the disease in question are compared with live controls with respect to screening histories. Such a design tends to yield estimates of relative mortality in voluntary participants compared with non-participants.This may bias results, since compliers and non-compliers may differ a priori in ways which are not related to screening but which nevertheless affect the risk of death from the disease. We present a simple method, employing external data from previously published randomized controlled trials of screening, of correction for this bias. We illustrate it by using data from a case-control study performed within the invited arm of the Malmö mammographic screening trial, a prospective study from the service screening programme in two counties in Sweden, and a matched case-control study of mammographic screening in Florence, Italy.