Study On The Related Factors Affecting The Positioning Error In CBCT-guided VMAT Of Cervical Cancer

Part one Study on the influence of registration mode,abdominal wall thickness and abdominal circumference on positioning error in radiotherapy for cervical cancerObjective:The purpose of this study is to explore the effects of different registration methods,abdominal wall thickness and abdominal circumference on the positioning error in CBCT-guided VMAT,so as to provide a scientific basis for clinical selection of a reasonable registration mode,and further calculate the planned target expansion range of patients with different abdominal wall thickness and abdominal circumference,so as to improve the accuracy of radiotherapy for obese patients.Methods:A retrospective analysis was made on 215 patients with cervical cancer who received radiotherapy in the affiliated Hospital of North Sichuan Medical College from June 1,2017 to October 1,2019.Automatic osseous registration(t+r),gray registration(t+r)and gray registration(t)were used for registration respectively.The patients were divided into groups according to different abdominal wall thickness and abdominal circumference,and the extension boundary of CTV-PTV is calculated according to the expansion formula.One-way ANOVA was used to compare the translation errors among the three registration methods,and LSD-t test was used to compare the differences between groups.The positioning errors of different abdominal wall thickness and abdominal circumference groups under three registration modes were compared and analyzed by multi-factor analysis of variance(ANOVA).Result:(1)in CBCT-guided VMAT,the registration translation error is the largest in the head-foot(Y-axis)direction,the second in the ventral dorsal(Z-axis)direction,and the smallest in the left and right(X-axis)direction.In the case of automatic grayscale registration(t),the X,Y and Z axes of CTV-PTV are suggested to be 3.68mm,7.14mm and 3.57mm,respectively;The abdominal wall thickness of>30mm group are 3.94mm,10.09mm,5.38mm,respectively;and that of abdominal circumference>900mm group are 4.05mm,10.0mm and 5.42mm,respectively.Under gray(t)registration,the boundary values of patients with positioning errors less than 5mm in three-dimensional direction on the X,Y and Z axes are 2.78mm,3.50mm and 2.64mm,respectively.(2)there were significant differences in the positioning errors of 215 patients under the three registration methods on the Y axis(F=4.52,P<0.05).There were significant differences in positioning errors among the three registration methods(X,Y,Z axis<5mm)in Y axis(F=4.11,P<0.05)and Z axis(F=3.34,P<0.05).The positional errors of patients with different abdominal wall thickness in the same registration mode were significantly different in X axis(F=32.98,P<0.05),Y axis(F=96.85,P<0.05)and Z axis(F=20.75,P<0.05).The positioning errors of the patients with the same abdominal wall thickness in different registration methods were significantly different on the Y axis(F=83.39,P<0.05).Under the same registration mode,the positioning errors of patients with different abdominal circumference were significantly different in X axis(F=377.70,P<0.05),Y axis(F=23.43,P<0.05)and Z axis(F=80.47,P<0.05).In the case of the same abdominal circumference,the positioning errors of patients with different registration methods were significantly different on the X axis(F=17.71,P<0.05),Y axis(F=15.74,P<0.05),Z axis(F=31.39,P<0.05).Conclusion:(1)CBCT-guided VMAT,gray registration(t)is the best way to register cervical cancer.(2)if there is no CBCT guide,the extension boundary of CTV-PTV should be 4mm in front and back,left and right,and it is safe to put 8mm in the direction of head and foot;Such as IGRT,It is recommended to put 3mm before and after,left and right,and 4mm in the direction of head and foot to ensure the accuracy of the target area.(3)the greater the abdominal wall thickness and abdominal circumference,the more obvious the difference in positioning.Different CTV-PTV expansion boundaries should be adopted among different groups to improve the accuracy of radiotherapy.For obese patients with abdominal wall thickness>30mm or abdominal circumference>900mm,the expansion boundaries are 5mm,11mm and 6mm on X,Y and Z axis,respectively.Part two Study on the effect of setting error on dose and clinical effect in radiotherapy of cervical cancerObjective:The purpose of this study is to explore the effect of positioning errors caused by different registration methods on the target area and the dose of endangered organs in radiotherapy for cervical cancer,to further verify the accuracy of the results of the previous stage,and to choose the best registration method.At the same time,compare the effective rate,adverse reaction occurrence and survival rate of IMRT and IGRT,and further clarify that reducing the positioning error is the key to achieve accurate radiotherapy.Methods:77 patients with cervical cancer were selected from 215 cervical cancer patients included in the above study,and the simulation plan were made according to the average positioning errors on the X,Y and Z axes under different registration methods,the DVH dose distribution parameters of 77 original plans and 231 simulation plans were recorded and analyzed.Statistics of the three simulation plans under different registration methods compared with the original plan,target dose(PTV D95,Dmin,Dmax,Dmean,GTV D95)and endangered organ exposure volume(bladder V40,rectal V40,small intestinal V30,left and right femoral head V40)change rate≥3%,≥5%.The single factor analysis of variance(ANOVA)was used to analyze the changes of the target dose and the exposure volume of the endangered organs of the three simulation plans compared with the original plan.At the same time,the clinical data,short-term effective rate,side effects of radiotherapy and survival rate of 82 patients treated in our hospital from January 1,2016 to December 30,2017 were analyzed retrospectively.Result:(1)in 77 patients,the change of PTV D95 and GTV D95≥5%in bone registration(t+r)is 30.49%and 20.34%,respectively.It is 17.07%and 15.25%respectively on the gray scale registration(t+r).It is 15.85%and 15.25%respectively on grayscale registration(t).The changes of≥5%in rectumV40,intestineV30 and L/R-femurV40 were all above 50%under the three registration modes.When the absolute value of the setting error is less than 5mm on the X,Y and Z axes.In 45 patients,the change of PTV D95 and GTV D95≥5%in bone registration(t+r)is 8.51%and 3.03%,respectively.It is 0%on grayscale registration(t+r)and grayscale registration(t).The changes of≥5%in rectumV40,intestineV30 and L/R-femurV40 decreased with the decrease of positioning error under the three registration methods.When the absolute value of setting error is less than 3mm in X,Y and Z axis,the change of PTV D95 and GTV D95 in bone registration(t+r)is 1.22%and 0%,respectively.It is 0%in grayscale registration.The bladder V40 with dose change≥5%is 12.2%,12.2%and 9.76%respectively under the three registration methods.The rectal V40 with dose change≥5%is 13.41%,13.41%and 10.98%respectively under three registration modes.Small intestine V30 and L/R-femur femoral head V40 with dose change≥5%also decreased significantly.(2)There was significant difference in the positioning error of the three registration methods on the Y axis(F=3.07,P<0.05).There was significant difference between the simulation plan made by the three registration methods and the original plan PTV D95(F=3.96,P<0.05),Among them,the simulation plan in which the positioning error was less than 5mm in X,Y and Z axis was significantly different from that of the original plan PTV D95(F=4.85,P<0.05)and GTV D95(F=3.21,P<0.05).There was no significant difference in OAR among different registration methods(P>0.05).(3)Short-term effective rate:the effective rate of IGRT group(CR+PR)was 95.2%,IMRT group was 88.1%.Acute toxicity:The incidence of urinary system,digestive system and myelosuppression in IGRT group was 35.7%,28.6%and 59.6%,respectively.IMRT group was 66.6%,61.9%and 76.2%,respectively.Chronic toxicity:the incidence of urinary system and digestive system in IGRT group was 16.7%and 11.9%respectively.IMRT group was 30.9%,23.8%,respectively.There was significant difference in the toxicity and side effects of urinary system and digestive system of grade II+III between the two groups(P<0.05).Conclusion:(1)the positioning error of different registration methods has a great influence on the target area and OAR radiation dose,so gray registration(t)is selected in clinical treatment.(2)the influence on the target area is relatively small when the absolute value of positioning error is less than 5mm in X,Y and Z axes,and the radiotherapy is more accurate when the error is less than 3mm.When the error is greater than 5mm,it must be re-positioned and manually fine-tuned in order to improve the accuracy of radiotherapy and ensure the safety of radiotherapy.(3)IGRT is essential for cervical cancer.Compared with IMRT,it can improve the accuracy of radiotherapy for advanced cervical cancer and significantly reduce the acute digestive and urinary side effects of Ⅱ and Ⅲ.