Randomization: Individual volunteers, with names entered successively on allocation lists. Although criticisms of the randomization procedure have been made, a thorough independent review found no evidence of subversion and that subversion on a scale large enough to affect the results was unlikely.
Exclusions: Few, balanced between groups.
Sample size: 25,214 study (100% screened after entry CBE) and 25,216 control.
Consistency of reports: Good.
Intervention: Annual two-view MMG and CBE for 4 to 5 years.
Control: Usual care.
Compliance: Initially 100%, decreased to 85.5% by screen five.
Contamination: 26.4% in usual care 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.
External audit: Yes. Independent, with analysis of data by several reviewers.
Follow-up duration: 25 years.
Relative risk of breast cancer death, screening versus control : 1.09 (95% CI, 0.80-1.49).
Comments: This is the only trial specifically designed to study women aged 40 to 49 years. Cancers diagnosed at entry in both study and control groups were included. Concerns were expressed prior to completion of the trial about the technical adequacy of the MMGs, the training of the radiologists, and the standardization of the equipment, which prompted an independent external review. The primary deficiency identified by this review was the use of the mediolateral view from 1980 to 1985 instead of the mediolateral oblique view, which was used after 1985. Subsequent analyses found the size and stage of the cancers detected mammographically in this trial to be equivalent to those of other trials. This trial and NBSS-2 differ from the other RCTs in the consistent use of adjuvant hormone and chemotherapy following local breast cancer therapy in women with axillary node-positive disease.