Objective To investigate the difference of translational error and rotational error in the ra-diotherapeutic positioning after breast cancer modified radical surgery by using the two fixation methods of the cervicothoracic thermoplastic membrane and body thermoplastic membrane based on conical beam CT (CBCT) images.Methods A total of 82 patients with radiotherapy after breast cancer modified radical sur-gery admitted and treated in this hospital from January 2022 to September 2023 were selected as the study subjects and divided into the cervicothoracic membrane group (using the cervicothoracic thermoplastic mem-brane,n=52) and body membrane group (using the body thermoplastic membrane,n=30) according to the radiotherapeutic fixation methods.All patients weekly conducted the CGCT scanning in the first time radio-therapy and radiotherapeutic period.The verification image conducted the retification with the planned image. The translational errors in the left and right direction (X),head and foot direction (Y) and abdominal and dor-sal direction (Z) were recorded,as well as the rotation errors around the axis under the two fixing methods,including the rotation error of the X-axis rotation direction (Rx),Y-axis rotation direction (Ry),and Z-axis rotation direction (Rz).The differences in positioning errors were compared between the two groups. Results There was no significant difference in X,Y,Z,Rx,Ry and Rx positioning error between the two groups after the first treatment,1-week treatment and 2-week treatment (P>0.05).After 3-week treatment,the positioning errors in X[0.21(0.12,0.27)mm vs. 0.22(0.20,0.35)mm],Y[0.20(0.11,0.24)mm vs. 0.25(0.16,0.37)mm],Z[0.15(0.08,0.25)mm vs. 0.20(0.15,0.29)mm],Rx[0.57(0.22,1.10)° vs. 1.00 (0.70,1.50)°],Ry[0.50(0.30,1.20)° vs. 1.10(0.60,1.40)°]and Rz[0.30(0.20,0.80)° vs. 0.90(0.40,1.50)°]in the cervicothoracic membrane group were smaller compared with the body membrane group (P<0.05).After 4-week treatment,the positioning errors in X[0.19(0.12,0.27)mm vs. 0.25(0.21,0.31)mm],Y[0.21(0.11,0.27)mm vs. 0.26(0.22,0.32)mm],Z[0.12(0.05,0.28)mm vs. 0.22(0.13,0.35)mm],Rx[0.80(0.49,1.10)°vs. 1.20(0.80,1.80)°],Ry[0.55(0.20,1.12)°vs. 1.10(0.80,1.30)°]and Rz[0.61 (0.29,1.10)°vs. 1.10(0.80,1.40)°]in the cervicothoracic membrane group were also smaller compared with the body membrane group (P<0.05).Conclusion The thecervicothoracic thermoplastic membrane could re-duce the positioning error in the breast cancer radiotherapy compared with the body thermoplastic membrane.