Background: Diabetes and the etiology of breast cancer are clearly associated. However, the impact of diabetes on prognosis is not yet understood. Therefore, we conducted a retrospective cohort study to examine the relationship between diabetes and patient outcomes in breast cancer patients. Methods: We investigated 332 Japanese women with breast cancer who underwent curative surgery at our hospital. Patients without sufficient clinical information including haemoglobin A1c (HbA1c) and those with an observation period of less than one year were excluded. Results: Among the patients examined, 106 had diabetes at the time of their breast cancer diagnosis. Among the 296 patients with invasive breast carcinoma, 36 patients developed distant metastases during the mean observation period of 45 months. Sixteen patients died due to breast cancer while 13 died of other causes. Multivariate analysis revealed that diabetes, tumor size and estrogen receptor (ER) status were independent factors related to distant metastasis-free survival (DMFS) (p=0.038, p<0.001 and p=0.006, respectively). Kaplan-Meier curve analysis revealed that diabetes negatively affected the outcomes of ER-negative breast cancer patients both in DMFS and OS (p=0.045 and p=0.029, respectively). Meanwhile, patient outcomes did not differ according to the level of HbA1c in diabetes patients. Conclusion: Patients with diabetes had a significantly shorter DMFS, and the negative effect of diabetes on patient outcomes was more evident in ER-negative breast cancer. Our data indicates the importance of primary prevention of diabetes for breast cancer patients.

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