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      Twenty five year follow-up for breast cancer incidence and mortality of the Canadian National Breast Screening Study: randomised screening trial

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          Abstract

          Objective To compare breast cancer incidence and mortality up to 25 years in women aged 40-59 who did or did not undergo mammography screening.

          Design Follow-up of randomised screening trial by centre coordinators, the study’s central office, and linkage to cancer registries and vital statistics databases.

          Setting 15 screening centres in six Canadian provinces,1980-85 (Nova Scotia, Quebec, Ontario, Manitoba, Alberta, and British Columbia).

          Participants 89 835 women, aged 40-59, randomly assigned to mammography (five annual mammography screens) or control (no mammography).

          Interventions Women aged 40-49 in the mammography arm and all women aged 50-59 in both arms received annual physical breast examinations. Women aged 40-49 in the control arm received a single examination followed by usual care in the community.

          Main outcome measure Deaths from breast cancer.

          Results During the five year screening period, 666 invasive breast cancers were diagnosed in the mammography arm (n=44 925 participants) and 524 in the controls (n=44 910), and of these, 180 women in the mammography arm and 171 women in the control arm died of breast cancer during the 25 year follow-up period. The overall hazard ratio for death from breast cancer diagnosed during the screening period associated with mammography was 1.05 (95% confidence interval 0.85 to 1.30). The findings for women aged 40-49 and 50-59 were almost identical. During the entire study period, 3250 women in the mammography arm and 3133 in the control arm had a diagnosis of breast cancer, and 500 and 505, respectively, died of breast cancer. Thus the cumulative mortality from breast cancer was similar between women in the mammography arm and in the control arm (hazard ratio 0.99, 95% confidence interval 0.88 to 1.12). After 15 years of follow-up a residual excess of 106 cancers was observed in the mammography arm, attributable to over-diagnosis.

          Conclusion Annual mammography in women aged 40-59 does not reduce mortality from breast cancer beyond that of physical examination or usual care when adjuvant therapy for breast cancer is freely available. Overall, 22% (106/484) of screen detected invasive breast cancers were over-diagnosed, representing one over-diagnosed breast cancer for every 424 women who received mammography screening in the trial.

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          Most cited references18

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          Swedish two-county trial: impact of mammographic screening on breast cancer mortality during 3 decades.

          To estimate the long-term (29-year) effect of mammographic screening on breast cancer mortality in terms of both relative and absolute effects. This study was carried out under the auspices of the Swedish National Board of Health and Welfare. The board determined that, because randomization was at a community level and was to invitation to screening, informed verbal consent could be given by the participants when they attended the screening examination. A total of 133 065 women aged 40-74 years residing in two Swedish counties were randomized into a group invited to mammographic screening and a control group receiving usual care. Case status and cause of death were determined by the local trial end point committees and, independently, by an external committee. Mortality analysis was performed by using negative binomial regression. There was a highly significant reduction in breast cancer mortality in women invited to screening according to both local end point committee data (relative risk [RR] = 0.69; 95% confidence interval: 0.56, 0.84; P < .0001) and consensus data (RR = 0.73; 95% confidence interval: 0.59, 0.89; P = .002). At 29 years of follow-up, the number of women needed to undergo screening for 7 years to prevent one breast cancer death was 414 according to local data and 519 according to consensus data. Most prevented breast cancer deaths would have occurred (in the absence of screening) after the first 10 years of follow-up. 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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            Canadian National Breast Screening Study-2: 13-year results of a randomized trial in women aged 50-59 years.

            Screening for breast cancer with mammography in women aged 50 years or more has been shown to reduce mortality from breast cancer. However, the extent to which mammography contributes to the reduction of mortality in women who also undergo physical examination of the breasts is not known. This study was designed to compare breast cancer mortality following annual screening consisting of two-view mammography and physical examination of the breasts with mortality following annual screening by physical examination only. Breast self-examination was taught to all participants. This trial randomly and individually assigned 39 405 women aged 50-59 years, recruited from January 1980 through March 1985, to one of the study arms. The women were followed by record linkage with the Canadian National Cancer Registry and National Mortality Database to December 31, 1993, and by active follow-up of breast cancer patients to June 30, 1996. Randomization achieved virtually equal distribution of demographic and breast cancer risk variables. At the first annual screen, 21% of the cancers found by mammography alone (in the mammography plus physical examination group) were 20 mm or more in size compared with 46% of those found by physical examination in the mammography plus physical examination group and 56% in the physical examination-only group. The corresponding percentages for screens 2-5 were 10%, 42%, and 50%, respectively. Screening detected 267 invasive breast cancers in the mammography plus physical examination group compared with 148 in the physical examination-only group. By December 31, 1993, 622 invasive and 71 in situ breast carcinomas were ascertained in the mammography plus physical examination group, and 610 and 16 were ascertained in the physical examination-only group. At 13-year follow-up, with 107 and 105 deaths from breast cancer in the respective groups, the cumulative rate ratio was 1.02 (95% confidence interval = 0.78-1.33). In women aged 50-59 years, the addition of annual mammography screening to physical examination has no impact on breast cancer mortality.
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              The Canadian National Breast Screening Study-1: breast cancer mortality after 11 to 16 years of follow-up. A randomized screening trial of mammography in women age 40 to 49 years.

              The efficacy of breast cancer screening in women age 40 to 49 years remains controversial. To compare breast cancer mortality in 40- to 49-year-old women who received either 1) screening with annual mammography, breast physical examination, and instruction on breast self-examination on 4 or 5 occasions or 2) community care after a single breast physical examination and instruction on breast self-examination. Individually randomized, controlled trial. 15 Canadian centers. 50 430 volunteers age 40 to 49 years, recruited from January 1980 to March 1985, who were not pregnant, had no previous breast cancer diagnosis, and had not had mammography in the preceding 12 months. Breast physical examination and instruction on breast self-examination preceded random assignment of 25 214 women to receive mammography and annual mammography, breast physical examination, and breast self-examination and 25 216 women to receive usual community care with annual follow-up. Verified breast cancer incidence and cohort mortality through 31 December 1993 and deaths from breast cancer through 30 June 1996. The 105 breast cancer deaths in the mammography group and 108 breast cancer deaths in the usual care group yielded a cumulative rate ratio, adjusted for mammography done outside the study, of 1.06 (95% CI, 0.80 to 1.40). A total of 592 cases of invasive breast cancer and 71 cases of in situ breast cancer were diagnosed by 31 December 1993 in the mammography group compared with 552 and 29 cases, respectively, in the usual care group. The expected proportions of nonpalpable and small invasive tumors were detected on mammography. After 11 to 16 years of follow-up, four or five annual screenings with mammography, breast physical examination, and breast self-examination had not reduced breast cancer mortality compared with usual community care after a single breast physical examination and instruction on breast self-examination. The study data show that true effects of 20% or greater are unlikely.
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                Author and article information

                Contributors
                Role: professor emeritus
                Role: data manager
                Role: professor emerita
                Role: statistician
                Role: senior scientist
                Role: professor
                Journal
                BMJ
                BMJ
                bmj
                BMJ : British Medical Journal
                BMJ Publishing Group Ltd.
                0959-8138
                1756-1833
                2014
                11 February 2014
                : 348
                Affiliations
                [1 ]Dalla Lana School of Public Health, University of Toronto, Toronto, Ontario M5T 3M7, Canada
                [2 ]Women’s College Research Institute, Women’s College Hospital, Toronto, Ontario M5G 1N8, Canada
                [3 ]Child Health Evaluative Services, The Hospital for Sick Children, Toronto, Ontario, Canada
                Author notes
                Correspondence to: A B Miller ab.miller@ 123456utoronto.ca
                Article
                mila016406
                10.1136/bmj.g366
                3921437
                24519768
                0c5dc7f8-6192-4f70-bbd9-070399c2dcbf
                © Miller et al 2014

                This is an Open Access article distributed in accordance with the Creative Commons Attribution Non Commercial (CC BY-NC 3.0) license, which permits others to distribute, remix, adapt, build upon this work non-commercially, and license their derivative works on different terms, provided the original work is properly cited and the use is non-commercial. See: http://creativecommons.org/licenses/by-nc/3.0/.

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