192P The association between endocrine therapy use and osteoporotic fracture among post-menopausal women treated for early-stage breast cancer in Ontario, Canada

Abstract

The use of endocrine therapy for early stage breast cancer, particularly aromatase inhibitor therapy has been associated with an increased risk of osteoporosis and fracture in clinical trials. We sought to validate this observation in real-world practice. We used health administrative data collected from post-menopausal women (aged ≥66 years) who were diagnosed with breast cancer and started on adjuvant endocrine therapy from 2005-2012 with follow-up until 2017. Patients were classified by use of either an aromatase inhibitor or tamoxifen and followed to measure an unadjusted cumulative 5-year incidence of developing an osteoporotic fracture. A multivariable analysis adjusting for age, charlson score, history of osteoporosis and/or osteoporotic fracture, previous use of steroids or a bisphosphonate and duration of endocrine therapy was completed using a cox-proportional hazards model. We identified 12,077 patients of whom 73% were treated with an aromatase inhibitor versus 27% with tamoxifen. The median age was 73 years (IQR 69-78). Our multivariable analysis did not demonstrate any significant difference in the rate of osteoporotic fracture between patients treated with an aromatase inhibitor or tamoxifen [Hazard ratio (HR)=1.09; 95% confidence interval (CI)=0.96-1.23, p-value=0.18]. The 5-year rate of osteoporotic fracture was 7.5% and 6.9% respectively. Factors associated with the development of osteoporotic fracture were included older age, higher charlson score, previous use of steroids or a bisphosphonate. The 5-year risk of osteoporotic fracture among older patients with pre-existing osteoporosis or fracture was 13.5% and 12.2% among patients using either an aromatase inhibitor or tamoxifen. Our study did not observe a significant difference in the rate of osteoporotic fracture among patients treated with an aromatase inhibitor versus tamoxifen. Our fracture rate was comparable to previous meta-analysis reports by the Early Breast Cancer Trialists’ Collaborative Group. In older patients (≥75 years) with pre-existing osteoporosis or fracture, we observed a high fracture risk warranting careful preventative management.