Objective To investigate the relationship between different body position fixation tech-niques,umbilical plane volume change(ΔV),body weight change rate(ΔW%),and radiotherapy setup errors in cervical cancer patients,and to provide recommendations for determining the margin of planning target vol-ume(MPTV).Methods A retrospective analysis was performed on the clinical data of 57 cervical cancer pa-tients who underwent radiotherapy at this hospital from June 2022 to May 2023.Patients were divided into the observation group(fixed with body membrane+thermoplastic pad,n=24)and the control group(fixed with body membrane alone,n=33)based on different positioning fixation methods.They were also further strati-fied by median BMI into BMI≥23.82 kg/m2 and BMI<23.82 kg/m2 patients.Setup errors in the left-right(X),cranio-caudal(Y),and anterior-posterior(Z)directions were recorded.Meanwhile,the umbilical plane volume on the first CT positioning image and the patient's body weight before positioning were recorded,as well as umbilical plane volume of cone-beam CT(CBCT)verification images during weekly radiotherapy and body weight before scan,the ΔV and ΔW%were calculated.Setup errors were compared between two groups,and correlations between ΔV,ΔW%and setup errors were analyzed in all patients,in two groups(the obser-vation group and the control group)and in two BMI subgroups.MPTV values in X,Y,and Z directions were calculated,and receiver operating characteristic(ROC)curve determined the cut off values of ΔV and ΔW%when setup errors met the department's MPTV criteria.Results Compared with the control group,the ob-servation group showed significantly smaller setup errors in X,Y,and Z directions(P<0.05).Both ΔV and ΔW%were positively correlated with setup errors in X and Y direction in all patients,patients in the control group,and patients with BMI≥23.82 kg/m2(P<0.05).In the observation group,ΔW%was positively corre-lated with setup errors in Z direction in patients with BMI<23.82 kg/m2(P<0.05);In the control group,ΔV and ΔW%were positively correlated with setup errors in X and Y direction in patients with BMI≥23.82 kg/m2.ROC curve analysis showed that when setup errors in Y direction reached the department's MPTV criteria(8.41 mm),the cutoff values ΔV and ΔW%were 8.045 cm2 and 4.12%,respectively.Conclusion The body membrane+thermoplastic pad fixation technique reduces setup errors and mitigates the impact of ΔV and ΔW%on setup errors in X and Y directions.When ΔV or ΔW%exceeds the cutoff values,increasing CBCT verification frequency and re-fabricating the body membrane are recommended.