BACKGROUND: Patients with breast cancer may have an increased incidence of atrial fibrillation (AF) because of the systemic inflammation induced by the cancer and side effects of treatments.

OBJECTIVE: The purpose of this study was to estimate the long-term incidence of AF in patients with breast cancer compared with the background population.

METHODS: We identified patients diagnosed with breast cancer from 1998 to 2015 by using nationwide registries. Female patients with breast cancer were matched (1:3) by age and sex with the background population. The long-term incidence of AF was estimated by cumulative incidence curves and multivariable Cox regression models.

RESULTS: We matched 74,155 patients with breast cancer with 222,465 patients from the background population. Breast cancer was associated with incident AF and the association differed between age groups (interaction analysis, P < .0001) and follow-up time periods. In patients younger than 60 years breast cancer was associated with increased incidence of AF during the first 6 months (hazard ratio [HR] 2.10; 95% confidence interval [CI] 1.25-3.44) and from 6 months to 3 years (HR 1.80; 95% CI 1.38-2.35). In patients older than 60 years, breast cancer was not associated with increased incidence of AF during the first 6 months (HR 1.13; 95% CI 0.95-1.34) and was associated with increased incidence of AF from 6 months to 3 years (HR 1.14; 95% CI 1.05-1.25).

CONCLUSION: The long-term incidence of AF was increased in patients with breast cancer and short-term incidence was increased in patients younger than 60 years and similar in patients older than 60 years compared with the background population.