Relationship between diabetes and risk of second primary contralateral breast cancer

Breast cancer survivors have a substantially higher risk of developing a second primary contralateral breast cancer (CBC) compared to the risk of breast cancer among women in the general population. While data regarding the relationship between diabetes and breast cancer incidence are inconsistent, diabetes is more clearly linked to an elevated risk of all-cause mortality among breast cancer survivors. However, no prior studies have assessed its impact on CBC risk. We assessed the relationship between diabetes, and CBC risk in a population-based nested case-control study consisting of women 40-79 years of age diagnosed with a first primary ER-positive invasive breast cancer. It included 322 women who developed a second primary CBC and 616-matched control women diagnosed only with a first breast cancer. We used conditional logistic regression to quantify associations between diabetes and CBC risk. Compared to women without a history of diabetes, diabetics had a 2.2-fold [95% confidence interval (CI) 1.3-3.6] increased risk of CBC. This risk was more pronounced among women diagnosed with their first breast cancer before age 60 years (odds ratio, OR = 11.5, 95% CI 2.4-54.5), compared to those diagnosed at age 60 years or older (OR = 1.5, 95% CI 0.8-2.7, P for interaction = 0.011). Diabetics diagnosed with breast cancer appear to have an elevated risk of CBC. This is the first study to report this relationship, but if confirmed efforts to insure that diabetic breast cancer survivors are carefully screened for second breast cancers may be warranted.