Effect of body mass index on tumor characteristics and disease-free survival in patients from the HER2-positive adjuvant trastuzumab trial N9831

BACKGROUND Data suggest that weight, and specifically body mass index (BMI), plays a role in breast cancer development and outcome. The authors hypothesized that there would be a correlation between BMI and clinical outcome in patients with early stage, human epidermal receptor 2 (HER2)-positive breast cancer enrolled in the N9831 adjuvant trial. METHODS Patients were grouped according to baseline BMI as follows: normal (BMI <25 kg/m²), overweight (BMI ≥25 kg/m² and <30 kg/m²), and obese (BMI ≥30 kg/m²). Disease-free survival (DFS) was estimated using the Kaplan-Meier method. Comparisons between treatment arms A, B, and C (chemotherapy with or without trastuzumab) were performed using a stratified Cox proportional hazards model. RESULTS Analysis was completed on 3017 eligible patients. Obese patients were more likely to be older and postmenopausal (P <.0001 for both), to have larger tumors (P =.002), and to have positive lymph nodes (P =.004). In the pooled analysis cohort, differences in DFS among the BMI groups were statistically significant (5-year DFS rate: 82.5%, 78.6%, and 78.5% for normal weight, overweight, and obese women, respectively; log-rank P =.02). The adjusted hazard ratio comparing the DFS of overweight women with the DFS of normal women was 1.30 (95% confidence interval, 1.06-1.61); and, comparing the DFS of obese women with the DFS normal women, the adjusted hazard ratio was 1.31 (95% confidence interval, 1.07-1.59). There were no statistically significant differences in DFS by weight group for women within any trial arm. CONCLUSIONS Patients with early stage, HER2-positive breast cancer and normal BMI had a better 5-year DFS compared with overweight and obese women. The current results indicated that adjuvant trastuzumab improves clinical outcome regardless of BMI.