The set-up errors and influencing factors of thoracic cancer patients receiving radiotherapy in different seasons in northwest China：a real world study

Abstract Objective: This study aims to measure the set-up errors in thoracic cancer patients undergoing VMAT radiotherapy and weekly cone-beam computed tomography (CBCT), and to evaluate the impact of different seasons on set-up errors. Methods: The CBCT verification results of 168 chest tumor patients in our department from November 2019 to 2021 to January 21 were collected as winter cohort, and a total of 840 sets of off-line CBCT images were collected. CBCT verification results for 139 patients from June 2021 to August 2022, and 717 sets of off-line CBCT images were collected.then the Mann-Whitney U test was performed.The PTV margin were then calculated according to the formula MPTV=2.5Σ+ 0.7δ. Gender, age, disease type, and arm position were also included for multivariate analysis. Results: Set-up error of summer vs.winter cohort, LR direction:1(1,2) vs.1(1,2) mm. SI direction: 2(1,3) vs. 2(1,3) mm,AP direction: 1(1,3)vs.1(1,2)mm. Rtn direction 0.3(0.1,0.55)°vs. 0.3(0.1,0.50)°. The PTV margins in the LR, SI, and AP directions were 4.18mm,4.85mm,4.59mm in the summer cohort,and 4.40mm,5.01mm,3.90mm in the winter cohort. Multivariate analysis showed that sex was statistically significant in the LR direction (b=0.164, t=1.993, p=0.048). The effect of the arm placement was statistically different in the SI direction (b= -0.46, t=4.84, P <0.001). Conclusion: It is found that for the patients with chest tumor fixation, the set-up error in the AP direction in the winter cohort was significantly smaller than that in the summer cohort. The conventional 5mm outer release boundary from CTV to PTV is sufficient for the summer cohort, but 4mm for the AP direction of the winter cohort. In the SI direction, the set-up error of the arm placed body side is significantly smaller than the arm raised on the forehead. Female patients greater set-up error in the LR direction than in male patients.