Objectives: To estimate the interval cancer incidence, its determinants and the episode sensitivity in the Norrbotten Mammography Screening Programme (NMSP).
Setting: Since 1989, women aged 40-74 years (n = 55,000) have been invited to biennial screening by the NMSP, Norrbotten county, Sweden.
Methods: Data on 1047 invasive breast cancers from six screening rounds of the NMSP (1989-2002) were collected. We estimated the invasive interval cancer rates, rate ratios and the episode sensitivity using the detection and incidence methods. A linear Poisson-model was used to analyse association between interval cancer incidence and sensitivity.
Results: 768 screen-detected and 279 interval cancer cases were identified. The rate ratio of interval cancer decreased with age. The 50-59 year age group showed the highest rate ratio (RR = 0.52, 95% CI 0.41-0.65) and the 70-74 year age group the lowest (RR = 0.23, 95% CI 0.15-0.36). The rate ratios for the early (0-12 months) and late (13-24 months) interval cancers were similar (RR = 0.18, 95% CI 0.15-0.22 and 0.20, 95% CI 0.17-0.24). There was a significantly lower interval cancer incidence in the prevalence round as compared with the incidence rounds. According to the detection method the episode sensitivity increased with age from 57% in the age group 40-49 years to 84% in the age group 70-74 years. The corresponding figures for the incidence method were 50% and 77%, respectively.
Conclusion: Our study showed an interval cancer incidence of 38% and the episode sensitivity of 62-73%, depending on the method of calculation. Our results are of clinically acceptable level and concert with the reference values of the European guidelines.