title: Number needed to screen: lives saved over 20 years of follow-up in mammographic screening.
abstract: OBJECTIVE
  To estimate the number needed to screen with mammography to save one life, based on a stated amount of screening activity and long-term follow-up for breast cancer death.
  
  
  METHODS
  A randomised controlled trial of mammographic screening for breast cancer, with 77,080 women invited to screening and 55,985 not invited. The invited group was offered screening for seven years. Follow-up continued for a total of just over 20 years.
  
  
  METHODS
  Number needed to screen for seven years to save one life over 20 years was calculated by dividing the number screened (not the number invited) by the total number of lives saved. Similarly, we calculated the number of mammographic examinations required to save one life.
  
  
  RESULTS
  We estimate that the number of women needed to screen for seven years to save one life over 20 years is 465 (95% CI 324-819). The number of mammographic examinations needed to save one life was 1499 (95% CI 1046-2642).
  
  
  CONCLUSIONS
  The number needed to screen to save one life is smaller than has been reported in the past. Mammographic screening is effective in absolute terms as well as relative. Long-term follow-up allowed us to estimate the absolute benefit with greater accuracy.
authors.affiliation: Mammography Department, Central Hospital, Falun, Sweden.
authors.forename: L
authors.initials: L
authors.lastname: Tabar
authors.forename: B
authors.initials: B
authors.lastname: Vitak
authors.forename: M F A
authors.initials: MF
authors.lastname: Yen
authors.forename: H H T
authors.initials: HH
authors.lastname: Chen
authors.forename: R A
authors.initials: RA
authors.lastname: Smith
authors.forename: S W
authors.initials: SW
authors.lastname: Duffy
mesh_terms: D000328:Adult; D000368:Aged; D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000453:epidemiology* / Q000401:mortality; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D008327:Mammography / Q000706:statistics & numerical data; D008403:Mass Screening / Q000379:methods / Q000706:statistics & numerical data; D008875:Middle Aged; D018401:Sample Size; D013548:Sweden / Q000453:epidemiology; D013997:Time Factors