Data source

The current study was based on data linkage of The Regional Breast Cancer Clinical Quality Registers of the Uppsala/Örebro, Stockholm-Gotland, and Northern regions of Sweden. These registries represent approximately 60 % of breast cancer patients in Sweden and contain information on date of diagnosis, tumor characteristics, primary treatment, and follow-up on all newly diagnosed breast cancer patients with less than 5 % missing information, in terms of diagnosis characteristics, of patients when validated against the National Swedish Cancer Register.

The Swedish Prescribed Drug Register contains information on all the prescribed medications dispensed in Swedish pharmacies since July 1, 2005. The database provides information on each dispensed drug including dates of dispensation, number of defined daily doses (DDD), and classification of the drugs based on the anatomic therapeutic chemical (ATC) system.

Individual-level information on socioeconomic and demographic factors was obtained from the Longitudinal integration database for health insurance and labor market studies (LISA) managed by Statistics Sweden. This nationwide database integrates existing data from labor market-, educational-, and social-sector registers and contains data from 1990 and onward on all individuals 16 years or older residing in Sweden and is updated yearly. Finally, the Swedish Inpatient Register contains information on hospital admission dates and diagnosis of diseases and has had complete national coverage since 1987.

Information from the above registers was linked using a ten-digit personal identifier numbers assigned for all persons registered in Sweden.