Study On Postoperative Radiotherapy Setup Error And CTV To PTV Extension Range Of Cervical Cancer

Objective:To measure the setup errors of patients with postoperative operation of cervical cancer during the treatment of intensity modulated radiation therapy（IMRT）,and to analyze the impact of setup errors on dose distribution of Clinical target volume（CTV）and surrounding normal tissue.Verify whether the margin of CTV to planning target volume（PTV）used in our hospital and calculated by empirical formula is reasonable,and explore a method to calculate the CTV to PTV outreach range.Methods:A total of 41 patients with cervical cancer confirmed by pathology as squamous carcinoma were selected from the radiotherapy center of our hospital from October 2019 to September 2020.All patients were treated with thermoplastic membrane fixation.CT simulation localization was performed in supine position.After the localized CT images were uploaded to the Monaco5.11.03 treatment planning system and reconstructed in 3D,and the same radiotherapy doctors will delineate The CTV,PTV and organs at risk were and administer the prescribed dose.The physicist sets up the radiation field,and calculates the dose distribution by IMRT,so as to achieve the optimal dose distribution in the target area.After the plan is confirmed by the physician in charge,the patient is treated.In the course of radiotherapy,patients received weekly CBCT scans,and the k V CBCT images were reconstructed,which were registered with planned CT images.The XVI system used the above image data for reconstruction to obtain coronal,sagittal and cross-sectional CT images.Image registration is the CBCT on the basis of the contact ratio of CT simulation images by the osseous registration（pubic symphysis,sacroiliac joint,flange on the bilateral iliac）with manual fine tuning.After two radiotherapy physicians confirmed that the image registration met the requirements,the XVI system automatically calculated the translation errors in the X axis（left and right）,Y axis（head and foot）and Z axis（abdomen and back）directions,and recorded the translation values respectively,namely the sett-up errors.Correction is performed by moving the position of the treatment bed.The setup error was measured once a week（only the setup error measurement data was used in this study after the setup and correction were measured respectively each time）,with 5 radiotherapy intervals to represent the setup error of 1 week,and a total of 5 setup errors were measured for each patient.In the treatment plan,the center point of the actual treatment field can be found according to the value of the set-up error.By moving the center point of the firing field of the original treatment plan（PLANA）to the center of the firing field of the actual treatment without changing the Angle,shape and weight of the firing field,the treatment plan（PLANB）combining the positioning error can be obtained.By comparing the dosimetric parameters of target area and organs at risk（OAR）of the simulation plan（PLANB）and the original plan（PLANA）,it was verified whether the margin of CTV to PTV used in our hospital was reasonable;Calculate M_PTV（CTV to PTV expansion value）according to the formula.Expand PTV again according to M_PTV,make a new plan,and get the original plan derived from the empirical formula（PLAN1）.Again,a simulation plan（PLAN2）combining the setup errors was obtained by planned transplantation.By comparing the dosimetric parameters of the target area and organs at risk of the simulation plan（PLAN2）and the original plan（PLAN1）,the rationality of the expansion range obtained by the empirical formula was verified,and the feasibility of the application of the empirical formula in intensities modulated radiotherapy for cervical cancer was discussed.This experiment was designed as a paired experiment,and the paired t-test or paired nonparametric Wilcoxon rank sum test was used for the two plans according to whether the difference met the normality.Through the method of CTV expansion/retraction,CTV was gradually expanded or retracted to form a simulated CTV,namely CTV_X.The dose and volume percentage of CTV_X were observed and compared until the V₉₅ of CTV_X.When V₉₅ of CTV_X<99%,it can be considered that the former CTVx can make the OAR receive the minimum irradiation,and at the same time,it is enough to overcome the influence of the set-up error on the quantity of the CTV,so as to obtain a reasonable value of CTV to PTV margin.Results:1.The systematic errors after the setup of the 41 cervical cancer patients were 0.10、-0.40、-0.30mm,and the random errors after the setup were 3.00、6.40、2.70mm;The systematic errors after correction were 0.00、-0.30、0.00mm,and the random errors after correction were 1.20、1.10、1.00mm.In this study,the sutup error was measured respectively after positioning and after alignment.Through comparison,it was found that although there was no statistical difference between the two,the sutup error after alignment was significantly smaller than that after alignment.2.For CTV,V₁₀₅,V₁₀₀,V₉₅,V₉₀,D₉₈,D₂ and D₅₀ in PLANB were all lower than PLANA.AND,there were statistically significant.The average values of each index were 4.75%,1.13%,0.44%,54.60c Gy,30.80c Gy and 4.90c Gy,respectively.V₁₀₅,V₁₀₀,V₉₅,D₉₈,D₂,and D₅₀ in PLAN2 were all lower than PLAN1,while V₉₀ had no change.The average values of each index were 3.37%,0.18%,0.05%,4.70c Gy,32.80c Gy and 2.0c Gy,respectively.However,there was no statistical difference in V₁₀₀,V₉₅ and D98,and the rest were statistically significant;For PTV,V₁₀₅,V₁₀₀,V₉₅,V₉₀,D₉₈,D₂ and D₅₀ in PLANB were all lower than that in PLANA,and PLANB was 3.34%,6.77%,2.81%,1.46%,373.00c Gy,31.70c Gy and 7.50c Gy lower than that in PLANA,respectively,with statistical significance;V₁₀₅,V₁₀₀,V₉₅,V₉₀,D₉₈,D₂ and D₅₀ in PLAN2were all lower than that in PLAN1,and PLAN2 was 4.83%,6.49%,2.21%,1.09%,295c Gy,32.7c Gy and 11.6c Gy lower than that in PLAN1,respectively,with statistical significance;For bladder,V₅₀,D₂ and D₅₀ in PLANB were all lower than PLANA,with the average values of each index being 2.64%,28.00c Gy and 17.25c Gy,respectively.V₄₀,V₃₀,V₂₀ and D₉₈were higher than PLANA,with the average values of each index being0.30%,0.25%,1.27%and 67.22c Gy,respectively.However,only V₅₀,V₃₀,V₂₀,D₉₈ and D₂ had statistical significance;V₅₀,V₄₀,V₂₀,D₂ and D₅₀ in PLAN2 were all lower than PLAN1,with the average values of each index being 1.62%,0.55%,0.09%,27.90c Gy and 15.20c Gy,respectively.V30 and D98 were higher than PLAN1,with the average values of each index being0.27%and 41.8c Gy,respectively.However,only V₅₀,V₂₀,D₉₈ and D₂ had statistical significance;For rectum,V₅₀,V₃₀,V₂₀,D₉₈,D₂ and D₅₀ in PLANB were all higher than that in PLANA.The average values of each index were0.32%,0.54%,2.26%,528.00c Gy,2.16c Gy and 12.83c Gy respectively,V₄₀was lower than PLANA,with a mean 0.26%lower;V₅₀,V₄₀,V₃₀,V₂₀,D₉₈,D₂ and D₅₀ in PLAN2 were all higher than that in PLAN1.The average values of each index were 0.62%,43.63%,1.50%,1.38%,231.70c Gy,26.70c Gy and 31.15c Gy respectively,but only D₉₈ had statistical significance;3.After calculation,the CTV to PTV margin calculated by the formula is 5㎜,10㎜,5㎜in X,Y,Z direction respectively.The V₉₅ of PLAN2（simulation plan）and PLAN1（original plan）is 99.83%and 99.88%,respectively,both of which are greater than 99%,and their margin is too large.V₉₅ of CTV1,CTV2,CTV3 and CTV4 in Plan2>99%,V₉₅ of while CTV5,CTV6<99%,that is,CTV4 corresponding to the outward expansion distance（3㎜,6㎜,3㎜in X,Y,Z direction respectively）can not only meet the dose requirements of the CTV,but also make the OAR avoid unnecessary irradiation,so the distance between CTV4 and PTV is relatively optimal.Conclusion:Under the condition of the CTV to PTV expansion range calculated by the formula and our hospital’s,the radiation dose of CTV decreased due to the setup error,and the radiation dose of normal organs was higher,but the actual radiation still met the requirements of the plan.The setup error caused the center to move dorsally,resulting in less dose distribution covering the bladder and more covering the rectum,which should be protected by measures.By combining the CTV expansion or retraction with the classical formula,it provides a new method to study the CTV expansion range.