Obesity, defined as a body mass index (BMI) ≥30 is an independent risk factor in
breast cancer and is correlated with shorter survival and enhanced recurrence rates. The present subgroup analysis of the German BRENDA-cohort aimed to investigate the correlation between BMI, recurrence-free survival (RFS) and adjuvant endocrine
therapy. In this subgroup analysis, 4,636 patients were retrospectively examined using multivariate analyses. Overall 3,759 (81.1%) patients had a BMI <30 (non-obese) and 877 (18.9%) a BMI ≥30 (obese). In the group of all 3,896 (84.0%) patients with
hormone-receptor-positive (HR+)
breast carcinomas a significant reduction in RFS was demonstrated for those who were obese (P = 0.002; HR = 1.45 (95% CI: 1.15-1.83)), also after adjustment for Nottingham Prognostic Index (NPI) (P = 0.028; HR = 1.30 (95% CI: 1.03-1.65)). In
hormone-receptor-negative (HR-) patients BMI had no influence on RFS (P = 0.380; HR = 1.20 (95% CI: 0.80-1.81)). Considering menopausal status, a significantly shorter RFS was seen in postmenopausal obese than in non-obese patients (P < 0.001; HR = 1.61 (95% CI: 1.24-2.09)), whereas the premenopausal patient group only showed a trend towards a shorter RFS (P = 0.202; HR = 1.44 (95% CI: 0.82-2.53)). The group of HR+ postmenopausal patients with normal or intermediate weight showed a non-significant statistical trend towards a survival benefit for
aromatase inhibitors (AI) compared to
tamoxifen (RFS: P = 0.486; HR = 1.29 (95% CI: 0.63-2.62), while obese patients tended to benefit more from
tamoxifen (RFS: P = 0.289; HR = 0.65 (95% CI: 0.29-1.45)). In accordance with recently published results we demonstrated a negative effect of a high BMI on outcome in primary
breast cancer. Furthermore the efficacy of AI seems dependent on BMI in contrast to
tamoxifen. Prospective studies to optimise the
therapy of obese
breast cancer patients are urgently needed.