Breast Cancer Risk Among Women With Type 2 Diabetes Differs By Ethnicity: The Multiethnic Cohort

Gertraud MASKARINEC, University of Hawaii, United States
JACOBS S. 1 , PARK S. 1 , CHRISTOPHER H. 2 , SETIAWAN W. 2 , LAURENCE K. 1 , LOIC L. 1 , WILKENS L. 1

1 Cancer Epidemiology, University of Hawaii Cancer Center, Honolulu, HI, USA
2 Department of Preventive Medicine, Norris Comprehensive Cancer Center, University of Southern California, Los Angeles, CA, USA

PURPOSE: Substantial research conducted primarily in white populations indicates a 20-30% elevated risk to develop breast cancer for T2D patients. This analysis investigated the association of T2D with breast cancer across five ethnic groups within the Multiethnic Cohort (MEC).
 
METHODS: In this cohort of 215,000 members aged 45-75 years at recruitment in 1993-1996, cancers were identified through tumor registries, deaths through vital records. T2D status was based on three questionnaires and three administrative data sources. Cox regression with age as the time metric was applied to estimate hazard ratios (HR) and 95% confidence intervals (CI). T2D status, i.e., a self-reported diagnosis confirmed by administrative data, was modeled as a time-varying exposure while adjusting for known confounders, including body mass index (BMI).
 
RESULTS: Among 100,855 (24,427 white, 20,025 African American, 7,596 Native Hawaiian, 26,396 Japanese American, 22,411 Latina) women, 6,557 breast cancer cases were identified after 14.8±4.1 years of follow-up. Of 17,991 self-reported T2D cases, 14,425 were confirmed by administrative data and 892 developed breast cancer. In models without BMI, T2D was significantly associated with T2D (HR=1.14; 95%CI 1.06-1.23), but including BMI lowered the estimate to 1.08 (95%CI 0.99-1.16). A strong association between BMI and breast cancer was observed in all ethnic groups except Latinas. The T2D-ethnicity interaction was borderline signficant (p=0.07); stratification by ethnicity showed elevated risks with and without adjustment for BMI among Latinas (HR=1.34; 95%CI 1.15-1.57 and HR=1.30; 95%CI 1.11-1.53) and African Americans (HR=1.18; 95%CI 1.02-1.37 and HR=1.30; 95%CI 1.11-1.53), but not in the other three ethnic groups.
 
CONCLUSIONS: As in previous reports, adjustment for BMI weakened the association of T2D with breast cancer and BMI was not a breast cancer predictor in Latinas. The signficant T2D association with breast cancer in Latinas and African Americans only is relevant for ethnic-specific prevention efforts.
 
FUNDING SOURCE: US NIH/NCI (U01CA164973)