PurposesThis study aims to examine whether diabetes has an impact on the use of surgery and adjuvant radiotherapy in treating women with localised breast cancer.MethodsWomen diagnosed with stage I–III breast cancer between 2005 and 2020 were identified from Te Rēhita Mate Ūtaetae—Breast Cancer Foundation New Zealand National Register, with diabetes status determined using New Zealand’s Virtual Diabetes Register. The cancer treatments examined included breast conserving surgery (BCS), mastectomy, breast reconstruction after mastectomy, and adjuvant radiotherapy after BCS. Logistic regression modelling was used to estimate the adjusted odds ratio (OR) and 95% confidence interval (95% CI) of having cancer treatment and treatment delay (> 31 days) for patients with diabetes at the time of cancer diagnosis compared to patients without diabetes.ResultsWe identified 25,557 women diagnosed with stage I–III breast cancer in 2005–2020, including 2906 (11.4%) with diabetes. After adjustment for other factors, there was no significant difference overall in risk of women with diabetes having no surgery (OR 1.12, 95% CI 0.94–1.33), although for patients with stage I disease not having surgery was more likely (OR 1.45, 95% CI 1.05–2.00) in the diabetes group. Patients with diabetes were more likely to have their surgery delayed (adjusted OR of 1.16, 95% CI 1.05–1.27) and less likely to have reconstruction after mastectomy compared to the non-diabetes group—adjusted OR 0.54 (95% CI 0.35–0.84) for stage I cancer, 0.50 (95% CI 0.34–0.75) for stage II and 0.48 (95% CI 0.24–1.00) for stage III cancer.ConclusionsDiabetes is associated with a lower likelihood of receiving surgery and a greater delay to surgery. Women with diabetes are also less likely to have breast reconstruction after mastectomy. These differences need to be taken in to account when considering factors that may impact on the outcomes of women with diabetes especially for Māori, Pacific and Asian women.