Use of Bisphosphonates and Risk of Breast Cancer in a French Cohort of Postmenopausal Women

Purpose To assess whether bisphosphonate (BP) use is associated with decreased breast cancer incidence in a cohort of postmenopausal women. Methods The study population included 64,438 postmenopausal women participating in the French E3N (Etude Epidemiologique aupres de femmes de la Mutuelle Generale de l'Education Nationale) prospective cohort, with data self-reported in biennial questionnaires matched with data from a drug reimbursement database. Exposure to BPs and the use of other osteoporosis treatments during follow-up were determined using reimbursement data. Other covariates (breast cancer risk factors, clinical risk factors for osteoporotic fractures, and bone mineral density surveillance) originated from the questionnaires. Hazard ratios (HRs) of breast cancer were estimated using Cox proportional hazards models, considering exposure as a time-varying variable. Results Over an average of 7.2 years of follow-up (2004 to 2011), 2,407 first primary breast cancer cases were identified. The HR of breast cancer associated with exposure to BPs was 0.98 (95% CI, 0.85 to 1.12). We found no effect modification by age, body mass index, time since menopause, use of hormone replacement therapy, use of calcium supplements, or use of vitamin D supplements. There was no heterogeneity across BP molecules and no trend according to cumulative dose, duration of use, or time since last use. We observed a decrease in breast cancer risk restricted to the year after treatment initiation (HR, 0.56; 95% CI, 0.36 to 0.87), which was likely explained by healthy screenee bias. Finally, we did not find any variation in HRs across breast carcinomas defined by their estrogen receptor or invasive or in situ status. Conclusion In our observational cohort of postmenopausal women observed from 2004 to 2011, BP use, likely prescribed for the management of osteoporosis, was not associated with decreased breast cancer incidence. (C) 2017 by American Society of Clinical Oncology