| Action Points
§ Explain that almost 30 years after initial mammography, screened women continue to have a significantly lower breast cancer mortality risk compared with unscreened women.
§ Note that while the relative benefit has remained virtually unchanged, the absolute benefit has increased over time because the number of deaths prevented increases with longer follow-up time. Almost 30 years after initial mammography, screened women continue to have a significantly lower breast cancer mortality risk compared with unscreened women, according to the latest results from a landmark Swedish study. Women randomized to screening mammography were 30% less likely to die of breast cancer as compared with women who did not undergo breast imaging. The relative benefit has remained virtually unchanged since Swedish investigators reported the first results (Lancet 1985;1(8433):829-832). However, the absolute benefit has increased over time, as reported in an article published online in Radiology. "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," Laszlo Tabar, MD, of Falun Central Hospital, and coauthors wrote in conclusion. The Swedish Two-County Trial was the first study to demonstrate a reduction in breast cancer mortality from mammographic screening alone. A consistent benefit in favor of screening has persisted in periodic follow-up reports. The subsequent reports after 15, 20, and 25 years of follow-up also showed a growing absolute benefit in terms of lives saved by screening mammography. "Because breast screening prevents deaths in the medium to long term, rather than in the immediate future, long-term follow-up (at least 15 years) is required to estimate the absolute number of breast cancer deaths prevented," Tabar and coauthors wrote in their introduction. The trial began in 1977, as women ages 40 to 74 living in two Swedish counties were organized into population clusters, and the clusters were randomly assigned to screening mammography or to usual care. When enrollment ended, the trial comprised 133,000 women, 77,000 randomized to screening mammography and 56,000 to usual care. The screening phase of the trial lasted for about seven years. Follow-up ended in 2005 in one of the two counties involved in the trial and in 2006 in the other county, resulting in total follow-up of 28 to 29 years. Breast cancer cases and deaths were determined by a local endpoint committee and by overview committee consensus data. By local adjudication, 1,426 breast tumors were detected in the mammography group, and 351 patients died of breast cancer. The authors reported that 498 (35%) tumors were symptomatic at diagnosis, accounting for 186 (53%) of breast cancer deaths in the group. In the control group, 367 breast cancer deaths occurred among the 56,000 patients in the group. The overview committee consensus analysis showed that 339 breast cancer deaths occurred in each arm of the trial. Both analyses showed a significant mortality reduction in women assigned to screening mammography. The local endpoint committee found a 31% reduction in the relative risk of breast cancer mortality in screened patients (RR 0.69, P<0.0001). Overview committee consensus data showed a 27% reduction in breast cancer mortality risk (RR 0.73,P=0.002). Reviewing the impact of screening on breast cancer mortality over time, Tabar and coauthors noted a 26% risk reduction by local adjudication after 10 years of follow-up, increasing to 30% after 15 years and remaining at 30% through 25 years of follow-up. The overview committee consensus data showed a 20% reduction in breast cancer mortality risk after 10 years, increasing to 27% after 15 years and remaining at that level through the remainder of follow-up. "At 29 years of follow-up, the number of women needed to undergo screening for seven years to prevent one breast cancer death was 414 according to local data and 519 according to consensus data," the authors wrote. "Most prevented breast cancer deaths would have occurred (in the absence of screening) after the first 10 years of follow-up." | 
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