Population-based cohort study in Denmark of risk of type 2 diabetes after breast cancer diagnosis

Studies of type 2 diabetes (T2D) risk after a breast cancer (BC) diagnosis had limitations regarding sample size and follow-up. A population-based cohort study in Denmark included 74,526 BC-survivors and for each survivor five BC-free women matched for age and region (n=372,630). The 5-year cumulative incidences of T2D were 3.8% (95% CI 3.7-3.9) in the BC cohort and 3.3% (3.3-3.4) in the control cohort. This risk elevation was attenuated over follow-up, from aHR 1.20 (95% CI 1.15-1.25) during the first five years after diagnosis to 1.09 (1.05-11.13) during the first fifteen years after diagnosis. Adjuvant endocrine therapy (aHR 1.14; 95% CI 1.10-1.19), aromatase inhibitors (1.25; 1.18-1.32), and less so tamoxifen (1.05; 0.99-1.11) were associated with elevated T2D risk in women with BC versus cancer-free women. Among BC patients, chemotherapy (aHR 1.10; 95% CI 1.03-1.17), right-sided radiotherapy (1.18;1.09-1.27), and left-sided radiotherapy (1.24; 1.15-1.33) were associated with increased T2D risk. The authors conclude that BC was associated with excess risk of T2D. The excess risk was temporary and related to BC treatment.