Abstract: Objective The aim of this study was to investigate the correlation between postural fixation and setup error in patients with different body mass index(BMI)during intensity-modulated radiotherapy after breast conserving surgery for breast cancer.Methods The clinical data of 117 patients with breast cancer after breast conserving surgery who received intensity modulated radiotherapy were retrospectively analyzed.The patients were divided into two groups A and B according to different body positions during radiotherapy.The patients in the group A(57 cases)were the vacuum bag group,and the patients in the group B patients(60 cases)were in the breast bracket group.According to the BMI index,patients were divided into group 1(＜18.5 kg/m²),group 2(18.5-23.9 kg/m²),group 3(24.0-27.9 kg/m²)and group 4(≥28 kg/m²).Results The positioning errors in the left and right(X-axis)and head-to-foot(Y-axis)directions of patients in the group A were smaller than those in the group B(P＜0.05).The positioning errors in X and Y directions of patients in the A3,A4 group were smaller than those in the B3,B4 group(P＜0.05).In the anterior-posterior direction(Z axis),there was no significant difference between the group A and group B(P＞0.05).Conclusion The BMI index of breast cancer patients receiving intensity-modulated radiation therapy after breast conserving surgery has an effect on the positioning error.The overweight patients(BMI≥24 kg/m²),vacuum bag fixation is more suitable.

Key words: Breast cancer, Body mass index, Body position fixation, Setup error