Background: Although research suggests that type II diabetes mellitus (DM-2) is associated with overall and breast cancer- specific decreased survival, most prior studies of breast cancer survival investigated the effect of preexisting DM-2 without assessing the effect of DM-2 diagnosed at or after breast cancer diagnosis. This study examined the relationship between DM-2 diagnosed before and after breast cancer diagnosis and overall survival. Methods: This study uses linked Department of Defense cancer registry and medical claims data from 9,398 women diagnosed with breast cancer between 1998 and 2007. Cox proportional hazards models were used to assess the association between DM-2 and overall survival. Results: Our analyses showed that women with DM-2 diagnosed before breast cancer diagnosis tended to have a higher risk of mortality compared with women without diabetes [HR = 1.17; 95% confidence interval (CI), 0.95-1.44] after adjustment for potential confounders. Similarly, patients diagnosed with DM-2 at or after breast cancer diagnosis had increased mortality compared with women without DM-2 (HR=1.39; 95% CI, 1.16-1.66). The similar tendency was also observed among most subgroups when results were stratified by race, menopausal status, obesity, tumor hormone receptor status, and stage. Conclusions: Using data from a health system that provides universal health care to its beneficiaries, this study showed an increased risk of death associated with DM-2, regardless of whether it was diagnosed before or at/after breast cancer diagnosis. Impact: These results suggest the potential effects of factors independent of the timing of DM-2 clinical diagnosis on the association of DM-2 with overall survival.
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