- Odds of triple-negative breast cancer (TNBC), rather than estrogen receptor-positive/human epidermal growth factor receptor 2-negative (ER+/HER2−) disease, appears greater among women with type 2 diabetes (T2DM), particularly with metformin use.
Why this matters
- Multiple meta-analyses have established an association between T2DM and breast cancer.
- This is the first study to examine the relationship by cancer subtype and explore the effect of antidiabetic agents.
- Findings suggest a need for more frequent breast cancer screening in T2DM.
- Retrospective multicenter population-based case-case study of 4557 patients with newly diagnosed breast cancer from 2 Surveillance, Epidemiology, and End Results (SEER) database sites: Seattle, Washington (2004-2015), and Albuquerque, New Mexico (2004-2012).
- Funding: NIH.
- History of T2DM (vs no history) was associated with 38% excess risk for TNBC (aOR, 1.38; 95% CI, 1.01-1.89) vs ER+/HER2− subtype.
- History of T2DM conferred no excess risk (vs ER+/HER2−) for:
- ER+/HER2+ subtype.
- HER2 overexpressing (ER−/progesterone receptor-negative and HER2+) subtype.
- Metformin use was only associated with excess risk for TNBC vs ER+/HER2− subtype:
- Use within 6 months of diagnosis (vs nondiabetic): OR, 1.54; 95% CI, 1.07-2.22.
- Use within 13-24 months of diagnosis (vs nondiabetic): OR, 1.80; 95% CI, 1.13-2.85.
- Mostly white population, limiting generalizability.
- Retrospective, observational design.