Human insulin to increase breast cancer risk in Taiwanese women with type 2 diabetes.

Abstract

26 Background: To investigate whether human insulin use may be associated with risk of breast cancer in Taiwanese women with type 2 diabetes. Methods: The reimbursement databases of all Taiwanese diabetic patients from 1996 to 2009 were retrieved from the National Health Insurance. An entry date was set at 1 January 2004 and a total of 482,065 women with type 2 diabetes were followed up for breast cancer incidence until the end of 2009. Incidences for ever-users, never-users and subgroups of human insulin exposure (using tertile cutoffs of time since starting insulin, cumulative dose and cumulative duration of insulin) were calculated and the multivariable-adjusted hazard ratios were estimated by Cox regression. Results: There were 59,830 ever-users and 422,235 never-users, with respective numbers of incident breast cancer of 559 (0.93%) and 4,711 (1.12%), and respective incidence of 207.7 and 215.1 per 100,000 person-years. The overall adjusted hazard ratios (95% confidence intervals) did not show a significant association with insulin [1.098 (0.994-1.212)]. However, significant trends for the different categories of the dose-responsive parameters were observed. Patients in the third tertiles consistently showed a significantly higher risk of breast cancer while compared to never-users: 1.263 (1.093-1.458), 1.339 (1.164-1.540) and 1.331 (1.158-1.531) for ≥ 67 months for time since starting insulin, ≥ 39,000 units for cumulative dose of insulin, and ≥ 21.8 months for cumulative duration of insulin, respectively. Conclusions: This study discloses a significantly higher risk of breast cancer associated with the use of human insulin, demonstrating a significant dose-responsive relationship.