Abstract: Objective To analyze the placement error of the abdominal tumor in children and the boundary of the target area by CBCT, in order to improve the accuracy of radiation therapy and improve the curative effect. Methods A total of 157 children aged 1-14 years old with abdominal tumors from March 2018 to February 2019 were enrolled in the Department of Radiotherapy of Peking Union Medical College Hospital. The data of 168 groups of CBCT (cone beam computed tomography) were collected during the whole treatment period. Calculate the error in three directions: vertical (VRT), longitudinal (LNG), and lateral(LAT). The planning target volume (PTV) external boundary is calculated by using the external boundary formula = 2.5∑+0.7δ. Results The statistical analysis of SPSS 21.0 showed that the placement errors in three directions were: VRT(-0.05±2.32)mm, LNG(0.11±2.67)mm, LAT(0.17±2.43)mm; calculate the PTV external boundary : VRT = 9.1 mm, LNG = 10.2 mm, and LAT = 9.4 mm. Conclusion The radiotherapy target area of children's abdominal tumors is VRT=9.1mm, LNG=10.2mm, and LAT=9.4mm is the best precision for radiotherapy, which provides a basis for improving the curative effect.

Key words: CBCT, Childhood abdominal tumor, Setup errors, Target area