Objective — Women with breast cancer and diabetes face worse outcomes than those with breast cancer without diabetes; however, the contribution of comorbidity to these disparities remains unclear. We evaluated the impact of diabetes on receipt of cancer treatments as well as mortality while accounting for other comorbidities.
Research Design and Methods — Ontario administrative databases were used to compare the rate of receipt of breast cancer treatments between women with and without diabetes. We also performed adjusted cause-specific hazard models to account for comorbidities when evaluating differences in treatments received and mortality outcomes between the two groups.
Results — Women with diabetes and stage III breast cancer were slightly less likely to receive chemotherapy (relative risk [RR] 0.93 [95% CI 0.89-0.97]), although this difference was not significant when we adjusted for comorbidities (adjusted hazard ratio [aHR] 1.03 [95% CI 0.93-1.13]). We saw similar trends for receipt of guideline-adherent radiotherapy (RR 0.97 [0.95-0.99], aHR 0.98 [0.94-1.02]). All-cause mortality was increased in women with diabetes after adjusting for comorbidities (aHR 1.16 [1.06-1.27]), but breast cancer-specific mortality was not increased overall. Women with a longer duration of diabetes and those with preexisting cardiovascular disease had increased all-cause and cancer-specific mortality.
Conclusions — Although cancer treatments received were similar between women with and without diabetes, breast cancer-specific mortality remains higher among women with diabetes who have longer diabetes duration or preexisting cardiovascular disease. This study uncovers new information about key risk factors for poorer prognosis in women with diabetes and breast cancer.