Differences in systemic treatments for breast cancer between patients with and without diabetes.

Aim: The objectives of this study are to investigate whether diabetes affects the systemic treatment of breast cancer.

Methods: Patients diagnosed with invasive breast cancer between 2005 and 2020 were identified from the Te Rēhita Mate Ūtaetae - Breast Cancer Foundation National Register. Logistic regression modelling was used to estimate odds ratios (ORs) with 95% confidence intervals (95% CIs) for the outcomes of endocrine therapy for estrogen receptor+/progesterone receptor+ cancer, targeted therapy for human epidermal growth factor receptor 2+ (HER2) cancer and chemotherapy in patients with breast cancer, comparing those with and without diabetes.

Results: Compared with patients without diabetes, patients with diabetes had lower probabilities of receiving endocrine therapy (64.2% vs 60.4%, p-value <0.001), HER2-targeted therapy (65.6% vs 54.8%, p-value <0.001) and chemotherapy (32.1% vs 20.4%, p-value <0.001). Most of the differences in receipt of endocrine therapy and HER2-targeted therapy between these two groups could be explained by adjustment for differences in age at diagnosis and comorbidity. The difference in usage of chemotherapy by diabetes status remained apparent after adjustment for other factors (OR 0.85, 95% CI 0.75-0.97), with a stronger difference in women with stage II breast cancer (OR 0.71, 95% CI 0.59-0.86) and in Pacific women (OR 0.70, 95% CI 0.51-0.94).

Conclusions: Women with diabetes are less likely to be treated with chemotherapy, and the difference is greatest in Pacific women and patients with stage II breast cancer. The lower usage of endocrine therapy and HER2-targeted therapy in patients with diabetes could be explained by the older age at diagnosis and more comorbidities.