Evaluation of the influence of positioning errors on target volume and spinal cord dose in intensity-modulated radiotherapy for lung cancer based on cone-beam CT

LU Cheng; HU Jiazhu; LIU Lingxiang

doi:10.3969/j.issn.1009-0460.2026.02.010

Abstract：Objective To quantitatively analyze the impact of positioning errors on dose coverage of tumor target areas during intensity-modulated radiotherapy(IMRT)for lung cancer based on cone-beam computed tomography(CBCT)images,and to assess the risk of under-target area irradiation and over-spinal cord irradiation caused by positioning errors.To provide dosimetric evidence for optimizing the clinical operation process of IMRT for lung cancer and enhancing the safety of treatment.Methods A total of 100 patients with non-small cell lung cancer who received IMRT treatment in our hospital from January 2024 to January 2025 were selected.All patients were placed in the supine position with body immobilization using a vacuum bag.CBCT scans were performed before and during the treatment(once every 5 treatments).Perform gray-scale registration on CBCT images and positioning CT images,and record the positioning errors of patients in the three directions of left and right(X-axis),head and foot(Y-axis),and front and back(Z-axis).The positioning error data was reverse-imported into the original treatment plan through the radiotherapy planning system(TPS)to generate an"error simulation plan".The dosimetric parameters of the target area in the original plan and the error simulation plan[including target area coverage(D95)]were compared and analyzed.The dose received by 95％of the target volume,the average dose of the target area(Dmean),the dose uniformity index of the target area(HI),and the dosimetric parameters of the spinal cord[the maximum dose of the spinal cord(Dmax),the volume of the spinal cord receiving a dose of ≥45 Gy(V45)].The patients were divided into the mild error group(absolute error ≤2 mm),the moderate error group(2 mm＜absolute error≤5 mm),and the severe error group(absolute error＞5 mm)based on the size of the positioning error.The incidences of target area underdose(D95＜95％of the prescribed dose)and spinal cord excess(Dmax＞45 Gy)were compared among the three groups.Results A total of 558 CBCT scans were completed for 100 patients,including 486 pre-treatment scans(87.10％)and 72 in-treatment scans(12.90％).The mean values and distributions of the positioning errors in the X,Y,and Z axes were as follows:The error range of the X-axis(left and right)was-4.35 to 3.92 mm,with an mean value of(1.88±1.26)mm;The Y-axis(head and foot)error range was-5.21 to 5.03 mm,with an average of(2.15±1.38)mm.The error range of the Z-axis(front and back)was-4.12 to 3.85 mm,with an average of(2.01±1.31)mm.The mild error group had 223 times(39.96％),the moderate error group had 214 times(38.35％),and the severe error group had 121 times(21.68％).Among them,the proportion of severe errors in the Y-axis direction was the highest(16.31％,91/558),and the proportion of severe errors in the X-axis direction was the lowest(8.96％,50/558).Compared with the original plan,the levels of planning target volume(PTV)D95,PTV Dmean,clinical target volume(CTV)D95 and gross tumor volume(GTV)D95 in the simulation plan were lower(P＜0.05),while the levels of PTV HI,spinal cord Dmaxand spinal cord V45 were higher(P＜0.05).The incidence of target area under-dose and spinal cord over-dose in the severe error group were both higher than those in the moderate error group and the mild error group(P＜0.05),and the moderate error group was significantly higher than the mild error group(P＜0.05).Pearson correlation analysis showed that the positioning errors in the X,Y,and Z axes were negatively correlated with PTV D95(r=-0.528,-0.682,-0.563,P＜0.001),and positively correlated with spinal cord Dmax(r=0.542,0.715,0.586,P＜0.001).Conclusion Positioning errors(especially in the Y-axis direction)during IMRT treatment for lung cancer can significantly increase the risk of under-target and spinal cord excess,and the degree of error is positively correlated with the risk.Strict control of positioning errors is of great clinical significance for ensuring target dose coverage and reducing spinal cord exposure.