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BACKGROUND: The effectiveness of mammography screening for women ages 40 to 49 years still is questioned, and few studies of the effectiveness of service screening for this age group have been conducted.

METHODS: Breast cancer mortality was compared between women who were invited to service screening at ages 40 to 49 years (study group) and women in the same age group who were not invited during 1986 to 2005 (control group). Together, these women comprise the Mammography Screening of Young Women (SCRY) cohort, which includes all Swedish counties. A prescreening period was defined to facilitate a comparison of mortality in the absence of screening. The outcome measure was refined mortality, ie, breast cancer death for women who were diagnosed during follow-up at ages 40 to 49 years. Relative risks (RRs) with 95% confidence intervals (CIs) were estimated.

RESULTS: There was no significant difference in breast cancer mortality during the prescreening period. During the study period, there were 803 breast cancer deaths in the study group (7.3 million person-years) and 1238 breast cancer deaths in the control group (8.8 million person-years). The average follow-up was 16 years. The estimated RR for women who were invited to screening was 0.74 (95% CI, 0.66-0.83), and the RR for women who attended screening was 0.71 (95% CI, 0.62-0.80).

CONCLUSIONS: In this comprehensive study, mammography screening for women ages 40 to 49 years was efficient for reducing breast cancer mortality.

Background Whether women in age 40-49 years should be invited to mammography screening or not is debated in many countries and a cost-effective alternative in countries with no screening in age 40-49 years could be selective screening i.e. inviting women at higher risk. In the current study relative effectiveness of mammography screening was estimated for subgroups based on the breast cancer risk factors parity, age at birth of first child and socioeconomic status (SES).

Methods The SCReening of Young women (SCRY) database consist of all women in age 40-49 years in Sweden in 1986-2005 and is split into a study and control group. The study group consists of women in areas where women age 40-49 years were invited to screening and the control group of women in areas where women 40-49 years were not. Rate ratio (RR) estimates were calculated for risk groups. Two exposures were considered; invitation to mammography screening and attendance.

Results There were striking similarities in the RR pattern for women invited to and attending in screening for all three risk factors and there was no statistically significant difference or trend in the RR by risk group. The RR estimates increased by increasing parity for parity 0 to 2 and ranged from 0.55 (95% CI 0.38-0.79) to 0.79 (95% CI 0.65-0.95) for women attending screening. The RR for white collar workers (low SES) was lower than for blue collar workers (high SES), 0.72 (95% CI 0.60-0.86) and 0.79 (95% CI 0.63-0.99) respectively for attending. For women 20-24 years at birth of first child RR was estimated at 0.73 (95% CI 0.58-0.91) for attending and estimates for other ages were similar.

Conclusion There was no statistically significant difference in relative effectiveness of mammography screening by parity, age at birth of first child or socio-economic status.

In cohort studies evaluating the effectiveness of an exposure such as an intervention or treatment on an outcome, self-selection in the form of non-compliance and contamination may lead to biased estimates of effectiveness. A previously published adjustment method by Cuzick et al for randomised controlled trials is further developed to a log-linear Poisson model for adjustment in cohort studies, allowing for estimation and testing also in subgroups with varying effectiveness. An example with both non-linear and log-linear interaction models for a Swedish mammography screening material with subgroups based on parity is presented.

Objectives: To estimate the level of overdiagnosis of all breast cancers and of invasive breast cancers in women aged 40–49 invited to the subsequent screening rounds in the Swedish service-screening programme 1986–2005.

Methods: To estimate the level of overdiagnosis in subsequent screening, the rate ratios (RR) of the breast cancer incidence in the study group (women in areas with screening in ages 40–49) and the control group (women in areas with no screening in ages 40–49) were calculated for all breast cancers and for invasive breast cancers. The RR estimates were adjusted for the prescreening difference in incidence between study and control group and for lead time.

Results: The prescreening incidence rate ratio was estimated at 0.92 (95% confidence interval [CI]: 0.88–0.97). The number of breast cancer cases and person-years were 6047 and 3.8 million, and 7790 and 5.2 million, in the study group and control group respectively during the study period. The RR estimate for all cancers was 1.01 (95% CI: 0.94–1.08) when adjusted for prescreening difference and a lead time of 1.2 years. The corresponding estimate for invasive breast cancers was 0.95 (95% CI: 0.88–1.02).

Conclusions: We found no significant overdiagnosis for women aged 40–49 in the Swedish service screening programme with mammography.