The association between pre-existing type 2 diabetes on cancer-related and all-cause mortality among women with breast cancer.

[PURPOSE] The objective of this study was to determine whether pre-existing type 2 diabetes is associated with an increased risk of breast cancer-related and all-cause mortality, compared with non-diabetes, and if the risk varies across glycated hemoglobin A1c (HbA1c) categories.

[METHODS] Using the Clinical Practice Research Datalink, we assembled a cohort of patients at least 18 years old, newly diagnosed with invasive breast cancer between 1998 and 2020, with follow-up until March 2021. Patients were followed from 3 months after breast cancer diagnosis until one of the study outcomes, end of registration with the general practice, or end of study. Multivariable Cox proportional hazards models, adjusted for 33 confounders, were fitted to estimate the hazard ratios (HRs) and 95% confidence intervals (CIs) of breast cancer-related and all-cause mortality, comparing patients with and without type 2 diabetes among patients with and without metastatic breast cancer. In a secondary analysis, HRs for the outcomes were estimated across glycated HbA1c categories.

[RESULTS] The cohort included 157,298 patients with incident breast cancer. Among patients with non-metastatic breast cancer (n = 125,268), type 2 diabetes was associated with a 12% increased risk of breast cancer mortality and 21% increased risk of all-cause mortality. Among patients with metastatic breast cancer (n = 32,030), type 2 diabetes was associated with a 22% increased risk of breast cancer mortality and a 24% increased risk of all-cause mortality. The secondary analysis showed that higher glycated HbA1c was associated with an increased risk, in both the non-metastatic and metastatic cohorts.

[CONCLUSION] In this large population-based cohort study, type 2 diabetes was associated with a greater risk of breast cancer-related and all-cause mortality, with an increased risk among patients with type 2 diabetes in the highest glycated HbA1c categories, compared to non-diabetes.