Objective: To compare the prognosis in women with interval breast cancer (cancer detected after a normal screening mammogram and before the next scheduled mammogram) with breast cancer detected among women not yet invited to mammography screening (non-screened).
Design: Population based observational study.
Setting: Norwegian breast cancer screening programme, implemented in different counties from 1996 to 2005.
Participants: 7116 women with a diagnosis of breast cancer at age 50 to 72 years; 1816 had interval breast cancer and 5300 had a diagnosis of breast cancer but had not yet been invited to screening.
Main outcome measures: Characteristics of the breast tumours, and survival of the women using Kaplan Meier curves and multivariable Cox proportional hazard models.
Results: Although interval cancers on average were slightly larger than the cancers in women not invited to screening, the histological type or status of axillary lymph nodes did not differ noticeably between the two groups. Among interval cancers, there were no appreciable trends in size, nodal status, grade, or hormone receptor positivity associated with time since the last normal mammogram as a marker of growth rate. After 10 years of follow-up, the survival rates were 79.1% (95% confidence interval 75.4% to 82.3%) among women with interval cancers and 76.8% (75.3% to 78.2%) among women in the non-screened cancer group (hazard ratio 0.98, 95% confidence interval 0.84 to 1.15; P=0.53). Analyses stratified by time since last normal mammogram, age at diagnosis, or screening round showed similar results.
Conclusion: The prognosis of women with interval breast cancers was the same as that of women with breast cancers diagnosed without mammography screening.