Positioning errors and margin of planning target volume of pelvic tumors patients in different fixed positions on a six-dimensional bed guided by cone beam CT
Objective To investigate the positioning errors and margin of planning target volume(MPTV)of pelvic tu-mors patients in different fixed positions on a six-dimensional bed guided by cone beam CT(CBCT).Method A total of 60 patients with pelvic tumors were divided into experimental group(n=30)and control group(n=30)according to differ-ent fixed positions.The experimental group was supine and fixed with vacuum negative pressure belt and fixation plate,while the control group was lateral and fixed with abdominal fixation device and positioning support.CBCT scans were performed in the two groups to obtain CBCT images and then registration was performed to obtain positioning errors in 6 directions,including left and right(X),head and foot(Y),front and back(Z),translation and rotation coronal position(Rx),sagittal position(Ry),and transverse position(Rz).The MPTV values for the translation direction(X,Y,Z)were cal-culated according to MPTV external boundary formula MPTV=2.5∑+0.7σ.Result The positioning errors in X and Y direc-tions of experimental group were significantly greater than those in control group,and the differences were statistically significant(P<0.01).The positioning errors in Z,Rx,Ry and Rz directions of experimental group were smaller than those in control group,and the differences were statistically significant(P<0.05).The MPTV values obtained by the experimental group were the smallest in the Y direction,and the MPTV values obtained by the control group were the smallest in the Z di-rection.Conclusion There are positioning errors in supine position with fixation device and lateral position with fixation device for pelvic tumors patients guided by CBCT on the six-dimensional bed.The selection of a patients'fixed position in clinical practice requires comprehensive analysis of patients'own conditions,degree of disease,psychology,location of disease and other factors to consider the impact of positioning errors.
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