Background and purpose: Radiotherapy is the main treatment for nasopharyngeal carcinoma. This study aimed to investigate the potential association between body mass index (BMI) and setup error during daily delivery of radiotherapy in the patients with nasopharyngeal carcinoma. Methods: This study enrolled 50 patients with nasopharyngeal carcinoma

who were treated with radical radiation therapy in Fudan University Shanghai Cancer Center from January 2017 to June 2017. The body weight was measured before radiation and 4 weeks post-radiation. Cone beam CT was performed at the same time point to obtain position data of the patients

including the patient’s head pin (LNG)

lateral (LAT)

dorsoventral (VRT) directions. Based on these analyses

we evaluated the accuracy of patient setup

as well as the adequacy of margins during target volume contouring. Results: The average body weights of patients before radiotherapy and 4 weeks post-radiotherapy were (53.27±3.79) and (49.37±3.29) kg

respectively. There was significant decrease of body weight during radiotherapy (P0.05). System error at LNG

LAT and VRT detections were 2.6

2.0 and 3.2 mm

respectively. The random error of LNG

LAT and VRT were 2.9

2.0 and 3.8 mm

respectively. BMI 24 was chosen to be the cutting point. In the subgroup with BMI≥24 kg/m2

the system errors at three-dimensional direction and horizontal rotation position were (2.80±0.35)

(2.30±0.17) and (3.90±0.42) mm

respectively. In the subgroup with BMI24 kg/m2

the system errors at three directions were (2.30±0.12)

(1.90±0.22) and (3.00±0.67) mm

respectively. There was significant difference in LNG

LAT and VRT between the two subgroups (P0.05). The correlation analysis revealed no significant association between setup values and BMI (P0.05). In other words

the setup error was higher in the subgroups with BMI≥24 kg/ m2. Decreasing BMI shifted the central point of patient setup towards ventral side and rotated the patient setup point reversely. Conclusion: There was no significant association between BMI and setup error before radiotherapy in patients with nasopharyngeal carcinoma. However

weight loss during radiotherapy influenced accuracy of patient setup. To ensure the accuracy of radiation

certain corrections have to be made during radiotherapy.