Breast Cancer Survivors Have Higher Diabetes Incidence

Joe Barber Jr, PhD

December 14, 2012

Postmenopausal breast cancer survivors have a modest but meaningful increase in the incidence of diabetes, according to the findings of a population-based observational study. However, diabetes incidence varied over time in breast cancer survivors.

Lorraine L. Lipscombe, MD, from Women's College Hospital in Toronto, the University of Toronto, and the Institute for Clinical Evaluative Sciences in Toronto, Ontario, Canada, and colleagues present their findings in an article published online December 13 in Diabetologia.

The potential link between cancer and diabetes has become increasingly accepted. "For instance, women with diabetes have an estimated 20% higher risk of postmenopausal breast cancer and up to a 50% increase in mortality after breast cancer diagnosis," the authors write. "This association is hypothesised to be due in part to the effects of insulin resistance and hyperinsulinaemia, which are known risk factors for both type 2 diabetes and cancer."

In the current study, the authors compared the incidence of diabetes in 24,976 women with newly diagnosed breast cancer and 124,880 age-matched women without breast cancer. The case and control patients were drawn from Ontario's population-based databases.

During the follow-up period, the overall incidence of diabetes was 9.8% for women with breast cancer vs 9.7% in the comparison group. The all-cause mortality rate was slightly higher in the in the women with breast cancer compared with those without breast cancer (11.6% vs 9.4%).

In the Cox proportional hazards regression analysis, the researchers adjusted for neighborhood income quintile, Charlson comorbidity index score, rural residence, and history of cardiovascular event, renal failure, and stroke or venous thromboembolism in the 5 years before cohort entry.

The researchers found that the interaction between breast cancer and diabetes varied over time, and therefore, they were unable to calculate a single hazard ratio (HR). Instead, the investigators calculated the HR at different points during follow-up. At 2 years, breast cancer survivors had a slightly elevated risk of diabetes compared with women without a history of breast cancer (HR, 1.07; 95% confidence interval [CI], 1.02 - 1.12). This risk increased over time to an HR of 1.21 (95% CI, 1.09 - 1.35) after 10 years of follow-up.

Women with breast cancer also had a higher risk for death after 1 year of follow-up (HR, 1.34; 95% CI, 1.24 - 1.44), and this risk remained significant after 12 years of follow-up (HR, 1.36; 95% CI, 1.21 - 1.54; P < .001).

Chemotherapy appeared to further increase the risk for diabetes for women with breast cancer, and the effect seemed to change with time. In subgroup analysis, the risk for diabetes among women with cancer who received chemotherapy peaked after 2 years of follow-up (HR, 1.24; 95% CI, 1.12 - 1.38) compared with women without breast cancer, and declined to an HR of 1.08 (95% CI, 0.79 - 1.48) after 10 years of follow-up. In contrast, the risk for diabetes among women with breast cancer who did not receive chemotherapy only reached statistical significance after 3 years of follow-up (HR, 1.06; 95% CI, 1.01 - 1.11). This risk increased over time to an HR of 1.24 (95% CI, 1.10 - 1.38) after 10 years of follow-up.

Study limitations, according to the authors, include potential selection bias, a lack of information on prescription use and cancer stage, and incomplete data on chemotherapy administration.

"In summary, this large, population-based study found a modest increase in the incidence of diabetes among postmenopausal breast cancer survivors compared with women without cancer," the authors write. "The reasons for this relationship are uncertain, and further research is necessary. In the meantime, greater diabetes screening and prevention strategies among breast cancer survivors may be warranted."

This study was supported by Cancer Care Ontario and the Ontario Institute for Cancer Research (through funding provided by the Ministry of Health and Long-Term Care and the Ministry of Research and Innovation of the Government of Ontario) and a Canadian Diabetes Association/Canadian Institute of Health Research Clinician Scientist Award, which also provided salary support for Dr. Lipscombe. Dr. Lipscombe also currently receives funding from a Canadian Institute of Health Research New Investigator Award. One coauthor was supported in part by a Career Investigator award from the Heart and Stroke Foundation. The other authors have disclosed no relevant financial relationships.

Diabetologia. Published online December 13, 2012. Abstract