title: Explaining the difference in prognosis between screen-detected and symptomatic breast cancers.
abstract: BACKGROUND
  We analysed 10-year survival data in 19,411 women aged 50-64 years diagnosed with invasive breast cancer in the West Midlands region of the United Kingdom. The aim was to estimate the survival advantage seen in cases that were screen detected compared with those diagnosed symptomatically and attribute this to shifts in prognostic variables or survival differences specific to prognostic categories.
  
  
  METHODS
  We studied tumour size, histological grade and the Nottingham Prognostic Index in very narrow categories and investigated the distribution of these prognostic factors within screen-detected and symptomatic tumours. We also adjusted for lead time bias.
  
  
  RESULTS
  The unadjusted 10-year breast cancer survival in screen-detected cases was 85.5% and in symptomatic cases 62.8%; after adjustment for lead time bias, survival in the screen-detected cases was 79.3%. Within narrow categories of prognostic variables, survival differences were small, indicating that the majority of the survival advantage of screen detection is due to differences in the distributions of size and node status.
  
  
  CONCLUSIONS
  Our results suggested that a combination of lead time with size and node status in 10 categories explained almost all (97%) of the survival advantage. Only a small proportion remained to be explained by biological differences, manifested as length bias or overdiagnosis.
authors.affiliation: CR-UK Centre for Epidemiology, Mathematics and Statistics, Wolfson Institute for Preventive Medicine, Charterhouse Square, London WC1M 6BQ, UK. p.allgood@qmul.ac.uk
authors.forename: P C
authors.initials: PC
authors.lastname: Allgood
authors.forename: S W
authors.initials: SW
authors.lastname: Duffy
authors.forename: O
authors.initials: O
authors.lastname: Kearins
authors.forename: E
authors.initials: E
authors.lastname: O'Sullivan
authors.forename: N
authors.initials: N
authors.lastname: Tappenden
authors.forename: M G
authors.initials: MG
authors.lastname: Wallis
authors.forename: G
authors.initials: G
authors.lastname: Lawrence
mesh_terms: D001943:Breast Neoplasms / Q000175:diagnosis / Q000000981:diagnostic imaging / Q000401:mortality* / Q000473:pathology; D055088:Early Detection of Cancer; D005260:Female; D006801:Humans; D008207:Lymphatic Metastasis; D008327:Mammography; D008875:Middle Aged; D011379:Prognosis; D015996:Survival Rate