Purpose — It has been suggested that type 2 diabetes may affect breast cancer prognosis, possibly due to increased diabetes-related comorbidity, or direct effects of insulin resistance and/or hyperinsulinemia. The purpose of this study was to examine the impact of diabetes on survival following breast cancer.
Methods — Using population-based health databases from Ontario, Canada, this retrospective cohort study compared deaths between women with breast cancer aged 55–79 years with diabetes and without diabetes. Women were followed for all cause mortality from breast cancer diagnosis until March 31st 2006.
Results — Of the 6,107 women with breast cancer, 1,011 had diabetes and 5,096 did not have diabetes. Women with diabetes were slightly older, were more likely to reside in a lower income neighborhood, and had greater comorbidity compared to women without diabetes. After a mean follow-up of 5.0 years and adjustment for age, prior mammograms and other covariates, mortality following breast cancer was significantly higher among women with versus without diabetes (hazard ratio, HR 1.39, 95% confidence interval, CI 1.22–1.59, P < 0.0001). The effect of diabetes on mortality was comparable to that seen in women with diabetes without breast cancer over a 7-year follow-up.
Conclusion — This study found that diabetes was associated with a close to 40% increase in mortality within the first 5 years following breast cancer, which was similar to that seen in women with diabetes without breast cancer. These findings suggest that early survival following breast cancer is reduced in women with diabetes, possibly due to diabetes-related causes.