Mammography Trials and Meta-Analyses
In Malmö, Sweden, a randomized trial to evaluate the efficacy of breast cancer screening was initiated in 1976, designated the Malmö Mammographic Screening Trial I. Approximately 42,000 women aged 45 to 69 were randomized on the basis of birth cohort (women born between 1927 and 1932) into study and control groups. The study group was invited to receive two-view mammography (craniocaudal and oblique views) at 18- to 24-month intervals for five rounds. CBE was not part of the screening regimen.
No overall reduction in deaths was observed in the study group compared with the control group at 8 and 11 years of follow-up, although a nonsignificant 20% lower breast cancer death rate was observed for women aged 55 and older. Among the explanations that have been offered for these results is the finding that approximately 25% of the control group was screened with mammography during the study period, and a similar percentage were nonattenders in the study group, thus diluting the differences between the original allocation. A second cohort of women born between 1933 and 1945, designated Malmö Mammographic Screening Trial II, were invited to be screened between 1978 and 1990.
The most recent update reports only results of the combined cohorts for women younger than age 50 at the time of randomization. The investigators observed a statistically significant 36% reduction in breast cancer mortality among the group invited to the screening.
The Stockholm randomized trial of breast cancer screening was initiated in 1981. Sixty thousand women aged 40 to 64 were randomized on the basis of birth date into a study group of approximately 40,000 women who would receive invitations to screening and a control group of 20,000 women. The study consisted of only two rounds of screening with single-view mammography (oblique view) without CBE at a 28-month interval and observed a nonsignificant 29% reduction in breast cancer mortality in the group invited to the screening.
The Gothenburg randomized trial of breast cancer screening was initiated in 1982. Approximately 52,000 women aged 39 to 59 years were randomized into groups to be invited and not to be invited to five screening rounds. The screening interval was 18 months, and usually women were screened with two-view mammography (craniocaudal and mediolateral oblique views), unless prior screening examinations indicated such low breast density that single-view mammography was justified (approximately 30% of mammograms). CBE was not part of the screening regimen. The investigators observed a 44% statistically significant reduction in breast cancer mortality among women aged 39 to 49 after 11 years of follow-up.
The Edinburgh trial was initiated in 1979 and was an evaluation of the efficacy of CBE and, initially, two-view mammography (depending on initial findings, single-view mammography was often performed in subsequent screening rounds). CBE was done annually and mammography every 2 years. Eighty-seven general practices representing approximately 45,000 women were randomized into study and control groups, which led to unanticipated confounding by socioeconomic status. Women aged 45 to 64 in the study group received invitations to four rounds of screening with mammography. At 7 years of follow-up, 17% fewer breast cancer deaths had occurred in the study group compared with the control group, a difference that was not statistically significant.