title: Number and characteristics of breast cancer cases diagnosed in four periods in the screening interval of a biennial population-based screening programme.
abstract: OBJECTIVE
  To describe the distribution and prognostic tumour characteristics of interval breast cancers diagnosed in four periods after index screen (1-6, 7-12, 13-18 and 19+ months) in a population-based screening programme inviting women aged 50-69 years to biennial screening.
  
  
  METHODS
  The Norwegian Breast Cancer Screening Programme (NBCSP) Methods: In all, 848 interval breast cancer cases were diagnosed in 437,235 screening examinations. The distribution and prognostic tumour characteristics of the interval cancers diagnosed in four periods in the screening interval will be described. Proportions and rates will be compared by chi(2)-test.
  
  
  RESULTS
  A total of 70% of the interval cancers in the NBCSP were diagnosed in the second year of the interval. Except for tumour size (P = 0.027), we found no evidence of adverse prognostic breast characteristics (grade, lymph node involvement, oestrogen and progesterone receptor positive) in invasive tumours diagnosed during the second versus the first year of the screening interval (Chi square P > 0.05 for all). The prognostic characteristics of the tumours did not differ by age groups. It was a decreasing interval cancer rate per 10,000 women-years by age.
  
  
  CONCLUSIONS
  The risk of interval cancer increases by time after index screen, and 70% of the interval cancers in the NBCSP were diagnosed in the second year of the interval. Prognostic histological tumour characteristics did not differ by time after index screen, thus mean sojourn time (tumour growth rate) seems important for stating an optimal screening interval in a population-based screening programme.
authors.affiliation: The Cancer Registry of Norway, Norway. sshh@kreftregisteret.no
authors.forename: Solveig
authors.initials: S
authors.lastname: Hofvind
authors.forename: Nils
authors.initials: N
authors.lastname: Bjurstam
authors.forename: Ragnhild
authors.initials: R
authors.lastname: Sørum
authors.forename: Hilde
authors.initials: H
authors.lastname: Bjørndal
authors.forename: Steinar
authors.initials: S
authors.lastname: Thoresen
authors.forename: Per
authors.initials: P
authors.lastname: Skaane
mesh_terms: D000368:Aged; D001943:Breast Neoplasms / Q000175:diagnosis / Q000453:epidemiology* / Q000473:pathology; D005260:Female; D006801:Humans; D008327:Mammography; D008403:Mass Screening / Q000379:methods*; D008875:Middle Aged; D009664:Norway / Q000453:epidemiology; D010865:Pilot Projects; D011379:Prognosis; D015397:Program Evaluation; D013997:Time Factors