Oral bisphosphonates and breast cancer in postmenopausal women
Data from the Women's Health Initiative (WHI)June 23, 2010
Study Design: Prospective cohort study
Study Eligibility: Postmenopausal women aged 50 to 79 years, accessible for follow-up, and with an estimated survival of 3 years or more
Enrollment: 2,816 Women's Health Initiative (WHI) participants who used oral bisphosphonates at the entry into the cohort and 151,952 participants who did not use bisphosphonates
Research Question: What is the relationship between bisphosphonate use and breast cancer risk in the WHI cohort of postmenopausal women?
Bisphosphonates are used to treat osteoporosis and to reduce skeletal-related events in cancer patients with bone metastases. From preclinical studies and clinical trials, research evidence suggests that intravenous bisphosphonates may reduce breast cancer recurrences. Low bone mineral density (BMD) is an indication for bisphosphonate use but is also associated with lower incidence of breast cancer. Studies on oral bisphosphonates have been limited due to the inability to control for bone mineral density (BMD) as a potential confounding factor.
Using data from the Women's Health Initiative (WHI), the current study evaluated the relationship between bisphosphonate use and breast cancer risk in postmenopausal women. To control for BMD as a confounding factor, adjustments for potential BMD differences between bisphosphonate users and nonusers were made using a hip fracture risk score. BMD was determined at the entry into the cohort and 5-year hip fracture risk was calculated using 11 clinical factors such as age, weight, height, race/ethnicity, self-reported physical activity, and smoking status. Participants also completed questionnaires on demographic, medical, and family history.
Those who used bisphosphonates were more likely to be white, older, have a family history of fracture, have a higher Gail model breast cancer risk, more likely to have prior benign breast biopsy, and have a family history of breast cancer. After a median follow-up of 8 years, invasive breast cancer incidence was significantly lower in bisphosphonate users compared to nonusers. The incidence of estrogen receptor(ER)-positive invasive breast cancer was also significantly lower in bisphosphonate users compared to nonusers. The incidence of ductal carcinoma in situ (DCIS) was found to be significantly higher in bisphosphonate users compared to nonusers. The clinical significance of increased DCIS incidence remains unresolved. Although the study had strengths that included the prospective design, racially diverse study population, comprehensive assessment of breast cancer risk factors, a major limitation included substantial differences in all of the baseline characteristics (e.g., age, education, alcohol use, physical activity, body mass index) between bisphosphonate users and nonusers. Residual confounding could have occurred and cloud the real relationship between bisphosphonate use and breast cancer risk. The study findings require prospective confirmation in other future studies.
Citations
Chlebowski RT, Chen Z, Culey JA et al. Oral bisphosphonate use and breast cancer incidence in postmenopausal women. J Clin Oncol 2010; DOI:10.1200/JCO.2010.28.2095.
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