ABSTRACT
Objective:To compare and analyze the differences in the setup accuracy of different immobilization method in breast cancer radiotherapy after breast-conserving surgery.Methods:A retrospective study was conducted on 60 patients who received radiotherapy after breast-conserving surgery from January to August, 2021. These patients were divided into two groups. One group consisted of 30 cases who were immobilized using a modified body thermoplastic membrane combined with a multifunction body board during the breast cancer radiotherapy and was called the modified body thermoplastic membrane group. The other group comprised 30 cases immobilized using a vacuum cushion during breast cancer radiotherapy and was referred to as the vacuum cushion group. The setup errors, 3D vector errors, the proportion of errors of > 5 mm, and the dosimetric differences in the planning target volume (PTV) and the clinical target volume (CTV) before and after simulated treatment bed moving (including the PTV_ V100, PTV_ V95, and CTV_ V95 before simulated treatment bed moving and the PTV_ V100 S, PTV_ V95 S, and CTV_ V95 S after simulated treatment bed moving) were compared between two groups. Moreover, for the modified body thermoplastic membrane group, the changes in the average setup errors at different radiotherapy stages were also analyzed. Results:A total of 369 cone-beam CT scans were conducted for 60 patients, including 195 CT scans for the modified body thermoplastic membrane group and 174 CT scans for the vacuum cushion group. The setup errors in the x, y, and z directions (right-left, anterior-posterior, and superior-inferior, respectively) of the modified body thermoplastic membrane group were (2.59±1.98) mm, (2.38±2.04) mm, and (1.45±1.16) mm, respectively, while those of the other group were (2.24±1.63) mm, (2.78±2.17) mm, and (2.70±1.88) mm, respectively. The 3D vector errors of both groups were (4.32±2.28) mm and (5.13±2.14) mm, respectively. Therefore, the setup error in direction z and the 3D vector error of the modified body thermoplastic membrane group were less than those of the vacuum cushion group ( t = -7.77, -3.41, P<0.05). Moreover, the proportion of setup errors of > 5 mm in the x direction of the vacuum cushion group was lower than that of the modified body thermoplastic membrane group ( χ2 = 7.13, P<0.05), while such proportion in the z direction of the modified body thermoplastic membrane group was lower than that of the vacuum cushion group ( χ2= 5.90, P<0.05). After the simulated treatment bed moving, the PTV_ V100 S of the modified body thermoplastic membrane group was better than that of the vacuum cushion group ( t = 2.47, P < 0.05). Furthermore, for the modified body thermoplastic membrane group, the setup errors in the x direction in the first week were higher than those in the 2-3 weeks and 4-5 weeks ( P<0.05). Conclusions:The modified body thermoplastic membrane combined with a multifunction body board yield better immobilization effects than a vacuum cushion. However, it produces high setup errors in the x direction in the first week of the radiotherapy, to which special attention should be paid.