In this population-based study, we evaluated the impact of obesity on presentation, diagnosis and treatment of breast cancer. Among all women diagnosed with invasive breast cancer in the canton Geneva (Switzerland) between 2003 and 2005, we identified those with information on body mass index (BMI) and categorized them into normal/underweight (BMI <25kg/m2), overweight (BMI ≥-<30kg/m2) and obese (BMI ≥30kg/m2) women. Using multivariate logistic regression, we compared tumour, diagnosis and treatment characteristics between groups. Obese women presented significantly more often with stage III-IV disease (adjusted odds ratio [ORadj]: 1.8, 95% CI: 1.0-3.3). Tumours ≥1cm and pN2-N3 lymph nodes were significantly more often impalpable in obese than in normal/underweight patients (ORadj 2.4, [1.1-5.3] and ORadj 5.1, [1.0-25.4], respectively). Obese women were less likely to have undergone ultrasound (ORadj 0.5, [0.3-0.9]) and MRI (ORadj 0.3, [0.1-0.6]) and were at increased risk of prolonged hospital stay (ORadj 4.7, [2.0-10.9]). This study finds important diagnostic and therapeutic differences between obese and lean women, which may impair survival of obese women with breast cancer. Specific strategies are needed to optimize the care of obese women with or at risk of breast cance
