Study On The Key Factors Affecting The Accuracy Of Dose Delivery In Radiotherapy

Radiotherapy is an important field in the application of nuclear technology.The main radiation device used in radiotherapy is electron linear accelerator,which is a relatively complex large-scale medical equipment.Its accuracy change will affect the accuracy of dose delivery and different planning complexity will also affect the accuracy of dose projection in radiotherapy;setup error will also affect the accuracy of dose delivery in radiotherapy.Therefore,experiments were designed to quantitatively analyze the key factors that affect the accuracy of radiation dose delivery,and then on this basis,discussed the methods to reduce the radiation dose delivery error of different patients.（1）The influence of accelerator hardware on the accuracy of radiation dose delivery is discussed.The experimental object is the accelerator gantry angle and collimator angle,which are the most basic and commonly used parameters;the material used in the experiment is a simple square field;the Gamma passing rate of the accelerator with different gantry angles and collimator angles is measured by using two-dimensional matrix Matri XX.It is found in the experiment that when the accelerator gantry angle and collimator angle are only studied,when they are turned to±13°and±3°respectively,the Gamma passing rate measured by two-dimensional matrix Matri XX is lower than 95%,which means that the collimator angle error has a greater impact on the accuracy of dose delivery than the gantry angle.When the gantry angle and collimator angle rotate at the same time,the Gamma passing rate is lower at the same angle,which indicates that the error of gantry angle or collimator angle may lead to larger dose deviation in practical application.Based on the above analysis,in the process of accelerator quality control,we should pay attention to the quality control of gantry angle and collimator angle;in practical application,we should make all quality control items meet the standard,and eliminate a short board.（2）The effects of different complexity plans on the accuracy of radiation dose delivery were compared.The experimental samples included 10 cases of nasopharyngeal carcinoma,14 cases of lung cancer and 20 cases of rectal cancer,a total of 44 radiotherapy plans.The main beam number,segment number and total MU number of these plans were recorded respectively,and then the Gamma passing rate was collected by using two-dimensional matrix Matri XX.First,analyze the difference of the number of segments and total MUs of different planning groups,and compare the statistical differences of the number of segments and total MUs of different complexity planning groups;then construct the multiple linear regression equation of Gamma passing rate and the number of segments and total MUs to analyze the factors affecting Gamma passing rate.Based on the above research results,the final purpose is to identify the parameters that affect the complexity of the radiotherapy plan,and to provide reference for the evaluation of radiotherapy plan.There were significant differences in the number of segments,total MUs and treatment time among the three groups;nasopharyngeal carcinoma had the most segments and total MUs,which meant that the complexity of nasopharyngeal carcinoma planning was the highest,followed by rectal cancer and lung cancer.The complexity of the plan is expressed by Gamma passing rate,which is the dependent variable and set as Y;the independent variables include the total number of MUs and the number of segments,which are set as X₁ and X₂ respectively;the multiple linear regression equation is established as:（?）=101.126-0.021X₁-0.003X₂.From the above multiple linear regression equation,it can be seen that the Gamma passing rate is mainly determined by the number of segments and the total MUs.With other independent variables unchanged,the more the number of segments,the lower the Gamma passing rate;the more the total MUs,the lower the Gamma passing rate.In the face of complex planning,the key to improve the accuracy of dose delivery is to reduce the number of segments and total MUs while not reducing the quality of planning.（3）The effect of patient positioning error on the accuracy of dose delivery was discussed.As an important part of the whole radiotherapy process,positioning is a necessary operation for patients before each treatment.Most patients with radiotherapy may have 10,20,30,33 or more times of treatment,and each treatment needs positioning.If each positioning has errors beyond the specified range,then the accumulated dose delivery errors in patients will not be accurately evaluated,and the effect of radiotherapy can not be guaranteed.Therefore,quantitatively analyze the influence of positioning errors on the accuracy of dose delivery is the key step to correct the setting error.In this study,5cases of nasopharyngeal carcinoma and 5 cases of rectal cancer were used to calculate their Gamma passing rates with different positioning errors.In NPC group,±1 mm of x-axis and y-axis errors could be detected by Matri XX;in rectal cancer group,±2 mm of x-axis and y-axis errors could be detected by Matri XX;when x-axis and y-axis errors were introduced at the same time,the average Gamma passing rate of nasopharyngeal carcinoma group and rectal cancer group was less than that of x-axis or y-axis alone,which indicates that positioning error has a great influence on the accuracy of dose delivery.Under the same standard conditions,NPC group plan is more sensitive to positioning error,and Gamma passing rate decreases faster,which indicates that different diseases have different tolerance for positioning error.Therefore,it is very important to place patients in strict accordance with the standard;the sense of responsibility of radiotherapy staff is very important;further research on the frequency of application of position verification tools for patients with different diseases before treatment is of great significance to ensure the accuracy of dose delivery.