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BACKGROUNDAlthough there is evidence for a reduction in breast carcinoma mortality with mammographic screening, some doubts have been expressed, and there is still uncertainty regarding the age specific effects.METHODSThe authors report on a randomized, controlled trial of mammographic screening for breast carcinoma that was conducted among 51,611 women (21,650 women who were invited to a screening [the study group] and 29,961 women in a control group) ages 39–59 years in Gothenburg, Sweden. Among women in the study group, the screening interval was 18 months. The screening phase of the trial took place in 1982–1991, and follow‐up for breast carcinoma mortality continued until December 31, 1996. Mortality from breast carcinoma was analyzed using a Poisson regression model. Overall and age specific effects of invitation to mammography screening on breast carcinoma mortality were calculated. Three mortality effects were estimated: the effect on deaths from breast tumors diagnosed during the screening phase of the trial, as assessed by an independent Endpoint Committee (the EPC evaluation model); the effect on deaths from breast carcinoma diagnosed during the screening phase of the trial, as determined by data from the National Cancer Registry and the National Cause of Death Register (the SCB evaluation model); and the effect on deaths from all breast carcinomas diagnosed up to December 31, 1996, as determined by the National Cancer Registry and the National Cause of Death Register (the SCB follow‐up model).RESULTSA nonsignificant, 21% reduction in the rate of mortality from breast carcinoma with invitation to screening was observed using the EPC evaluation model (relative risk [RR], 0.79; 95% confidence interval [95% CI], 0.58–1.08; P = 0.14); and a borderline significant, 23% rate reduction was observed using the SCB follow‐up model (RR, 0.77; 95% CI, 0.60–1.00; P = 0.05). Age specific analyses yielded greater mortality rate reductions for the groups of women ages 39–44 years, 45–49 years, and 55–59 years, but there was no mortality rate reduction in the group of women ages 50–54 years. The effects of invitation to mammographic screening on the incidence of lymph node‐positive disease closely paralleled the effects of invitation on breast carcinoma mortality. The effect on breast carcinoma mortality was consistent with the effect on all‐cause mortality, suggesting no bias in classification of cause of death. Breast carcinoma incidence in the study group was almost identical to the incidence in the control group after trial by screening had ended in the control group (RR, 0.98; 95% CI, 0.88–1.09; P = 0.7).CONCLUSIONSThe current results support the commonly observed 20–30% reduction in breast carcinoma mortality with invitation to screening. The impression that screening is less effective in women younger than 50 years may be an oversimplification. Age specific effects should be a target for further research. Cancer 2003;10:2387–96. © 2003 American Cancer Society.DOI 10.1002/cncr.11361

Randomised controlled trials have shown that the policy of mammographic screening confers a substantial and significant reduction in breast cancer mortality. This has often been accompanied, however, by an increase in breast cancer incidence, particularly during the early years of a screening programme, which has led to concerns about overdiagnosis, that is to say, the diagnosis of disease that, if left undetected and therefore untreated, would not become symptomatic. We used incidence data from two randomised controlled trials of mammographic screening, the Swedish Two-county Trial and the Gothenburg Trial, to establish the timing and magnitude of any excess incidence of invasive disease and ductal carcinoma in situ (DCIS) in the study groups, to ascertain whether the excess incidence of DCIS reported early in a screening trial is balanced by a later deficit in invasive disease and provide explicit estimates of the rate of 'real' and non-progressive 'overdiagnosed' tumours from the study groups of the trials. We used a multistate model for overdiagnosis and used Markov Chain Monte Carlo methods to estimate the parameters. After taking into account the effect of lead time, we estimated that less than 5% of cases diagnosed at prevalence screen and less than 1% of cases diagnosed at incidence screens are being overdiagnosed. Overall, we estimate overdiagnosis to be around 1% of all cases diagnosed in screened populations. These estimates are, however, subject to considerable uncertainty. Our results suggest that overdiagnosis in mammography screening is a minor phenomenon, but further studies with very large numbers are required for more precise estimation. IntroductionRandomised controlled trials have shown that the policy of mammographic screening confers a substantial and significant reduction in breast cancer mortality [1][2][3]. There is continuing interest in the human costs associated with the mortality benefit, in particular, whether overdiagnosis occurs in breast cancer screening and, if so, its magnitude [4,5]. In this context, overdiagnosis means the diagnosis of cancer as a result of screening, usually histologically confirmed, that would not have arisen clinically during the lifetime of the host had screening not taken place.When a mammographic screening programme is initiated, usually a large increase in breast cancer incidence is observed in the early years of the programme, and a relatively small increase later [4,6]. This in itself is not sufficient to imply overdiagnosis, for the following reasons: 1. In most parts of the world, breast cancer incidence was increasing prior to the epoch of mammography. Thus at least part of any excess incidence observed in the screening epoch is probably due to an existing increasing trend in incidence. 2. In addition, the early diagnosis of cancers due to lead time may exacerbate the underlying temporal increase by bringing forward in time future higher rates of disease. 3. In relation to this, screening also causes an artificial increase in age-specific ...