2.2. Group definitions

We calculated BMI as weight in kilograms divided by height in square meters. BMI was categorized according to WHO standard: normal weight, BMI < 25.0 kg/m²; overweight, 25.0≤BMI < 30.0 kg/m²; and obesity, BMI≥30.0 kg/m². Patient's weight and height were recorded before surgery. Three groups were divided according to BMI when breast cancer diagnosis: normal weight group (474), overweight group (351), and obesity group (192). Furthermore, a stratified analysis about relationship between BMI and prognosis of breast cancer was conducted in line with menopausal status when breast cancer diagnosed. Menopausal status was defined by 1 year of amenorrhea, or previous bilateral oophorectomy. In the present study, a total of 416 breast cancer patients were postmenopausal, and 601 breast cancer patients were premenopausal. The differences in clinicopathologic characteristics and 5-year breast cancer outcomes between 3 groups were compared.