Stage-specific incidence and survival of breast cancer in Norway: The implications of changes in coding and classification practice.

To describe the association between coding and classification practices and observed stage-specific incidence and survival trends in Norway over time. We identified all women diagnosed with invasive breast cancer in the period between 1980 and 2015. Changes in the coding and classification of breast cancer in the study period were described, and stage-specific incidence rates and relative survival were calculated. A total of 90 362 women were diagnosed with primary breast cancer, stage I-IV, or unknown stage, in the study period. Stage-specific incidence was significantly influenced by changes in coding practice, classification systems and the implementation of the screening program. These changes have mostly affected the proportion of stage I and "unknown", but also stages II, III and IV. The proportion of stage I showed a clear increase during the implementation period of the national screening program, and was most pronounced within the age group 50-69. Stage-specific trends for relative survival were less influenced by changes in coding and classification of stage. Our study showed that the stage-specific incidence trends in Norway were influenced by changes in the coding and classification practice. These findings should be taken into consideration in future research and evaluation related to stage-specific trends and stage migration of breast cancer in Norway.