Abstract: Objective To analyze the conventional segmental positioning error of lung cancer with Halcyon and the outspread boundary of the target area, so as to improve the precision and efficacy of radiotherapy in patients with lung cancer with Halcyon. Methods Totally 142 patients from May 2019 to August 2020 in the radiotherapy department of Peking Union Medical College Hospital were selected in this study and 3 192 registration images were collected for data analysis of the whole course of treatment, and the errors of elevation direction (VRT), inlet and outlet direction (LNG) and left-right direction (LAT) were calculated. The outer boundary formula of target area=2.5∑+0.7 and combined with the ICRU50 and ICRU62 report analysis results. Results By SPSS 21.0 statistical analysis, the positioning errors of the three directions were as follows: VRT(0.07±1.59)mm, LNG(-0.15±4.40)mm, and LAT (0.08±3.50)mm. According to the analysis of ICRU50 and ICRU62 reports, the target margin is VRT=3.69 mm,LNG=6.60 mm and LAT=5.04 mm. Conclusions With conventional segmental treatment model, the boundary of the exotherm of the lung cancer patients in the target area of the Halcyon medical electron linac was VRT=3.69 mm, LNG=6.60 mm and LAT=5.04 mm, which provided the basis for the exotherm of the clinical target margin.

Key words: conventional segmentation of lung cancer, Halcyon, CBCT, positioning error, target margin