Objective      To investigate the best position in image-guided radiotherapy(IGRT) for uterine cervix cancer(UCC).       Methods      Based on body mass index (BMI)

all the patients were divided into BMI≥24 group (9 patients) and BMI＜24 group (21 patients).All the patients were scanned by cone beam computerized tomography(CBCT)

including 68 scans in prone position and 73 scans in supine position.The scanned images were analyzed by grayscale color automatic registration to get the setup errors before correction in X

Y and Z directions. The best position in IGRT for UCC was studied

and the margins of the best position were calculated by formula 2.5Σ+0.7δ.       Results      Without the disturbance of BMI

the setup errors had no significant difference between supine position and prone position (P0.05). In the group of BMI＜24

the setup errors in X and Y directions had no significant difference between supine position and prone position(P0.05)

but the supine position error was -0.112±0.041 cm and the prone position error was 0.028±0.024 cm in Z direction

with significant differences (t=-3.186

P=0.002)

and the margins of prone position were 0.668

1.206

and 0.335 cm. In the group of BMI≥24

the setup errors in X and Y directions had no significant difference between supine position and prone position(P0.05)

but the supine position error was 0.061±0.045 cm and prone position was -0.26±0.031 cm in Z direction

with significant difference (t=5.832

P0.001)

and the margins of supine position were 0.313 cm

1.663 cm

and 0.501 cm.       Conclusion      The group of BMI＜24

the setup errors of prone position are significantly less than those of supine position in Z direction

while in the group of BMI≥24

the setup errors of supine position are significantly less than those of prone positionin Z direction.Between the 2 group the setup errors have no statistical difference in X

Y directions.