Type 2 diabetes as a predictor of survival among breast cancer patients: the multiethnic cohort.

PURPOSE: The purpose of the study was to investigate the association of type 2 diabetes (T2D) with survival of breast cancer (BC) patients across five ethnic groups within the Multiethnic Cohort study.

METHODS: Between recruitment in 1993-1996 and 2013, 7570 incident BC cases were identified through SEER cancer registries in Hawaii and California. T2D diagnosed before BC was ascertained in 1013 women from self-reports and confirmed by administrative data sources. Covariate information was collected by questionnaire. Cox regression analysis with age as the time metric and BMI as time-varying exposure was applied to estimate hazard ratios (HR) and 95% confidence intervals (CI) for BC-specific and all-cause survival while adjusting for known prognostic factors.

RESULTS: In total, 2119 all-cause and 730 BC-specific deaths were recorded with corresponding 5-year survival rates of 86 and 93%. T2D was not a significant predictor of BC-specific survival (HR 0.84; 95% CI 0.65-1.09), but mortality was 36% lower for those with < 7 years of T2D than a longer history of T2D. On the other hand, all-cause mortality was higher in women with T2D (HR 1.23; 95% CI 1.08-1.40), especially in women with T2D of ≥ 7 years duration (HR 1.27; 95% CI 1.07-1.49). In women receiving none or either chemotherapy or radiation but not both, T2D predicted higher all-cause mortality (Pinteraction  = 0.004). Variations in the association of T2D with mortality across ethnic groups were small.

CONCLUSIONS: T2D was associated with higher all-cause but not BC-specific mortality among women with BC in the Multiethnic Cohort study. However, T2D affected survival in cases who did not receive both radiation and chemotherapy.