Objective To evaluate the clinical application of Catalyst system in patient positioning during breast cancer radiotherapy,and to analyze its correlation with age and body mass index (BMI).Methods Twenty-four patients with breast cancer who were admitted to our hospital from May to August,2016 were enrolled as subjects.For all patients,auxiliary positioning was made by the optical surface imaging system (CRad Catalyst) before each treatment.The kV-kV imaging was executed weekly to verify positioning.Age,BMI,and setup errors of the two systems in the anterior-posterior (AP),superior-inferior (SI),and left-fight (LR) directions were recorded and analyzed by independent samples t-test and Pearson correlation analysis.Results The C-Rad Catalyst system had a significantly larger setup error in the AP direction than the kV-kV imaging (0.22±0.17 vs.0.18±0.13 cm,P＜0.05).There were no significant differences in setup errors in the SI or LR direction between the two systems (0.23±0.18 vs.0.19±0.15 cm,P＞0.05;0.28±0.28 vs.0.20±0.15 cm,P＞ 0.05).Age and BMI of patients had significant impacts on the C-Rad Catalyst system but the kV-kV imaging (P＞0.05):there were significant differences in setup errors in the AP and SI directions between patients ≤44 years of age and those 45-59 years of age (all P＜0.05);there were significant differences in setup errors in the AP and LR directions between patients ≤44 years of age and those ≥60 years of age (all P＜0.05);there was a significant difference in setup error in the LR direction between patients 45-59 years of age and those ≥ 60 years of age (P＜0.05);there was a significant difference in setup error in the SI direction between patients with BMIs of＜ 25 and ≥ 25 kg/m2 (P＜ 0.05).For patients ≥ 60 years of age,setup error of the C-Rad Catalyst system in the SI direction was correlated with age (r=-0.496,P＜0.05).For patients with BMI of＜25 kg/m2,setup error of the C-Rad Catalyst system in the AP direction was correlated with BMI (r=-0.445,P＜ 0.05).For patients with a BMI of ≥ 25 kg/m2,setup error of the C-Rad Catalyst system in the SI direction was correlated with BMI (r=-0.252,P＜0.05).Conclusions There is significant difference in setup error in the AP direction between the C-Rad Catalyst system and the kV-kV imaging.Age and BMI have impacts on patient positioning by the C-Rad Catalyst system.