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Breast Cancer Screening (PDQ®): Screening - Health Professional Information [NCI] - Appendix of Randomized Controlled Trials

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NBSS-2, Canada 1980 [15]

Age at entry: 50 to 59 years.
Randomization: Individual volunteer (see NBSS-1).
Exclusions: Few, balanced between groups.
Sample size: 19,711 study (100% screened after entry CBE) and 19,694 control.
Intervention: Annual two-view MMG and CBE.
Control: Annual CBE.
Compliance: Initially 100%, decreased to 86.7% by screen five in the MMG and CBE group. Initially 100%, decreased to 85.4% by screen five in the CBE only group.
Contamination: 16.9% of the CBE only group.
Cause of death attribution: Death certificates, with review of questionable cases by a blinded review panel. Also linked with the Canadian Mortality Data Base, Statistics Canada.
Analysis: Follow-up.
External audit: Yes. Independent with analysis of data by several reviewers.
Follow-up duration: 11 to 16 years (mean 13 years).
Relative risk of breast cancer death, screening versus control (95% CI): 1.02 (0.78–1.33).
Comments: This trial is unique in that it compares one screening modality to another, and does not include an unscreened control. Regarding criticisms and comments about this trial, see NBSS-1.

Stockholm, Sweden 1981 [16]

Age at entry: 40 to 64 years.
Randomization: Cluster by birth date. There were two subtrials with balanced randomization in the first and a significant imbalance in the second with 508 more women in the screened group than the control.
Exclusions: Inconsistently reported.
Sample size: Declined from 40,318 to 38,525 in intervention group and rose from 19,943 to 20,978 in control, between published reports.
Consistency of reports: Variable.
Intervention: Single-view MMG every 28 months × 2.
Control: MMG at year 5.
Compliance: 82% screened.
Contamination: 25% of women entering the study had MMG in the 3 years before entry.
Cause of death attribution: Linked to Swedish Cause of Death Registry.
Analysis: Evaluation, with 1-year delay in the posttrial MMG in control group. Follow-up analysis as part of the Swedish meta-analysis.[5]
External audit: No.
Follow-up duration: 8 years.
Relative risk of breast cancer death, screening versus control (95% CI): 0.80 (0.53–1.22).
Comments: There are concerns about randomization, especially in the second subtrial, about exclusions, and about the delay in control group MMG. Inclusion of these data in the Swedish meta-analysis resolves many of these questions.

Gothenberg, Sweden 1982

Age at entry: 39 to 59 years.
Randomization: Complex; cluster randomly assigned within birth year by day of birth for older group (aged 50–59 years) and by individual for younger group (aged 39–49 years); ratio of study to control varied by year depending on MMG availability (randomization took place 1982–1984).
Exclusions: A similar proportion of women were excluded from both groups for prior breast cancer diagnosis (1.2% each).
Sample size: Most recent publication: 21,650 invited; 29,961 control.
Consistency of reports: Variable.
Intervention: Initial two-view MMG, then single-view MMG every 18 months × 4. Single-read first three rounds, then double-read.
Control: Control group received one screening exam approximately 3 to 8 months after the final screen in study group.
Cause of death attribution: Linked to Swedish Cause of Death Registry; also used an independent endpoint committee.
Analysis: Both evaluation and follow-up methods.[5]
External audit: No.
Follow-up duration: 12 to14 years.
Relative risk of breast cancer death, screening versus control (95% CI): Aged 39 to 59 years: 0.79 (0.58–1.08) [evaluation]; 0.77 (0.60–1.00) [follow-up].
Comments: No reduction for women aged 50 to 54 years, but similar reductions for other 5-year age groups.
Conclusions: Delay in the performance of MMG in the control group and unequal numbers of women in invited and control groups (complex randomization process) complicates interpretation.
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Last Updated: February 25, 2014
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