Diabetes mellitus and breast cancer: a retrospective population-based cohort study.
OBJECTIVE
Evidence suggests that women with type 2 diabetes may be at increased risk of breast cancer, possibly due to chronic exposure to insulin resistance and/or hyperinsulinemia. The purpose of this study was to compare the incidence of breast cancer in postmenopausal women with and without diabetes.


METHODS
Using population-based validated health databases from Ontario, Canada, this retrospective cohort study compared breast cancer incidence between women, aged 55-79 years, with newly diagnosed diabetes (n=73,796) to women without diabetes (n=391,714).


RESULTS
Women with diabetes were slightly older, were more likely to reside in a lower income neighborhood, had greater comorbidity, and had more annual physician visits than women without diabetes. After 2.2 million person-years of follow-up from 1994 to 2002, breast cancer incidence was 2.97/1000 person-years in the diabetes group and 2.75/1000 person-years in the non-diabetes group. After adjustment for age and income, there was a significant increase in breast cancer among women with diabetes (hazard ratio, HR, 1.08, 95% confidence interval, CI, 1.01-1.16, p=0.021).


CONCLUSIONS
This study found a small but significant increase in incident breast cancer in a predominantly postmenopausal population of women with diabetes, when compared to women without diabetes. These results support the possibility that insulin resistance or some other aspect of type 2 diabetes may promote breast cancer, and may further direct treatment and prevention strategies.
Institute for Clinical Evaluative Sciences, M4N 3M5, Toronto, Ontario, Canada. Lorraine.Lipscombe@ices.on.ca
Lorraine L
LL
Lipscombe
Pamela J
PJ
Goodwin
Bernard
B
Zinman
John R
JR
McLaughlin
Janet E
JE
Hux
D000367:Age Factors; D000368:Aged; D001943:Breast Neoplasms / Q000150:complications* / Q000175:diagnosis / Q000453:epidemiology / Q000378:metabolism; D015331:Cohort Studies; D048909:Diabetes Complications / Q000175:diagnosis* / Q000453:epidemiology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D015994:Incidence; D008875:Middle Aged; D016016:Proportional Hazards Models; D013997:Time Factors; D016896:Treatment Outcome