The predictive impact of body mass index on the efficacy of extended adjuvant endocrine treatment with anastrozole in postmenopausal patients with breast cancer: an analysis of the randomised ABCSG-6a trial

Background: We investigated whether body mass index (BMI) can be used as a predictive parameter indicating patients who benefit from extended aromatase inhibitor (AI) treatment. Methods: The ABCSG-6a trial re-randomised event-free postmenopausal hormone receptor-positive patients from the ABCSG-6 trial to receive either 3 additional years of endocrine therapy using anastrozole vs nil. In this retrospective analysis, we investigated the prognostic and predictive impact of BMI on disease outcome and safety. Results: In all, 634 patients (177 normal weight, 307 overweight, and 150 obese) patients were included in this analysis. Normal weight patients with additional 3 years of anastrozole halved their risk of disease recurrence (disease-free survival (DFS) HR 0.48; P=0.02) and death (HR 0.45; P=0.06) and had only a fifth of the risk of distant metastases (HR 0.22; P = 0.05) compared with normal weight patients without any further treatment. In contrast, overweight+obese patients derived no benefit from additional 3 years of anastrozole (DFS HR 0.93; P = 0.68; distant recurrence-free survival HR 0.91; P = 0.78; and OS HR 0.9; P = 0.68). The possible predictive impact of BMI on extended endocrine treatment could be strengthened by a Cox regression interaction model between BMI and treatment (P = 0.07). Conclusion: Body mass index may be used to predict outcome benefit of extended AI treatment in patients with receptor-positive breast cancer.