title: Impact of screening mammography on mortality from breast cancer before age 60 in women 40 to 49 years of age.
abstract: BACKGROUND
  Whether screening mammography programs should include women in their 40s is controversial. In Canada, screening of women aged 40-49 years has not been shown to reduce mortality from breast cancer. Given that screening mammography reduces mean tumour size and that tumour size is inversely associated with survival, the lack of benefit seen with screening is puzzling and suggests a possible adverse effect on mortality of mammography or subsequent treatment (or both) that counterbalances the expected benefit derived from downstaging.
  
  
  METHODS
  We followed 50,436 women 40-49 years of age until age 60 for mortality from breast cancer. Of those women, one half had been randomly assigned to annual mammography and one half to no mammography. The impact of mammography on breast cancer mortality was estimated using a left-censored Cox proportional hazards model.
  
  
  RESULTS
  Of 256 deaths from breast cancer recorded in the study cohort, 134 occurred in women allocated to mammography, and 122 occurred in those receiving usual care and not allocated to mammography. The cumulative risk of death from breast cancer to age 60 was 0.53% for women assigned to mammography and 0.48% for women not so assigned. The hazard ratio for breast cancer-specific death associated with 1 or more screening mammograms before age 50 was 1.10 (95% confidence interval: 0.86 to 1.40).
  
  
  CONCLUSIONS
  Mammography in women 40-49 years of age is associated with a small but nonsignificant increase in the risk of dying of breast cancer before age 60. Caution should be exercised when recommending mammographic screening to women before age 50.
authors.affiliation: Dalla Lana School of Public Health, University of Toronto, Toronto, ON. ; Women's College Research Institute, Women's College Hospital, Toronto, ON.
authors.forename: S A
authors.initials: SA
authors.lastname: Narod
authors.affiliation: Women's College Research Institute, Women's College Hospital, Toronto, ON.
authors.forename: P
authors.initials: P
authors.lastname: Sun
authors.affiliation: Dalla Lana School of Public Health, University of Toronto, Toronto, ON.
authors.forename: C
authors.initials: C
authors.lastname: Wall
authors.affiliation: Dalla Lana School of Public Health, University of Toronto, Toronto, ON.
authors.forename: C
authors.initials: C
authors.lastname: Baines
authors.affiliation: Dalla Lana School of Public Health, University of Toronto, Toronto, ON.
authors.forename: A B
authors.initials: AB
authors.lastname: Miller
keywords: Breast cancer; mammography; mortality; screening