Number and characteristics of breast cancer cases diagnosed in four periods in the screening interval of a biennial population-based screening programme. 
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.  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.  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.  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.