Influence Of Leaf Transmission And The Feasibility Of Bias Dose In Sequential Radiotherapy

Background:Intensity modulated radiotherapy（IMRT）is the main technical means of clinical radiotherapy.MLC is one of the core hardware of IMRT.Its performance parameters and the accuracy of TPS modeling are related to the accuracy of radiotherapy dose,and MLC leaf transmission is the key parameter of TPS modeling.Studying the influence of the change of this parameter on radiotherapy planning can help clinical physicists to find the potential MLC faults of medical linear accelerator and TPS in time.Sequential radiotherapy is a commonly used radiotherapy method in clinic.the conventional planning method has some disadvantages,such as poor target conformal,low work efficiency,inconvenient evaluation and so on.The Bias Dose is a new technical method,which can overcome the above shortcomings,but its feasibility and advantages need to be further verified.Purpose:To verify the effect of MLC leaf transmission on radiotherapy planning and to verify the feasibility and advantages of Bias Dose in sequential radiotherapy planning,to provide more convenient planning and evaluation methods for clinic.Methods:10 cases of brain metastases,10 cases of esophageal cancer and 10 cases of prostate cancer were selected.Firstly,the influence of MLC leaf transmission on plan optimization was verified in the first course plan design.The first course plan was designed with the default value of 0.005 at the time of TPS installation,named FT5,and the plan was transplanted to the folder with leaf transmission 0.0025 and 0.0075respectively.The dose distribution of each group named FT25 and FT75;respectively was compared with that of FT5,and the dose change trend was analyzed.A two-dimensional QA plan was made for FT5,and the plan was transplanted into a model with a leaf transmission of 0.0025 and 0.0075,and the dose distribution of the QA plan was calculated directly.Execute the FT5 plan on the linear accelerator and measure the dose distribution.The measured data were compared with the QA plan calculated by the planning system,and theγpassing rate was calculated to compare the accuracy of TPS dose calculation.With FT5 as the background dose,the conventional method and Bias Dose method were used to design the radiotherapy plan.The plan generated by the general method is named Normal,Bias Dose method and the plan generated by the general method is named Bias.The dose distributions of the two groups were compared and the advantages and disadvantages of the two methods were analyzed.All data were analyzed by SPSS22 software.Results:In the study of MLC leaf transmission,based on the data of FT5 group,there was no significant difference in all the evaluation indexes of brain metastasis of lung cancer.Among the types of esophageal cancer,D₉₈,D_mean and TC in FT25 group were higher than those in FT5 group,and there was significant difference between D₉₈ and FT5group,and there was no significant difference in other parameters.In prostate cancer samples,the average rectal dose of FT25 group was lower,and the difference was statistically significant.With the increase of leaf transmission,MU showed a decreasing trend,the difference was statistically significant.There was no significant difference in other parameters.In the study of Bias Dose,the dose distribution of target and endangered organs was evaluated based on Normal group.Among the brain metastatic diseases of lung cancer,the D₂ of PGTV target and the D_mean of PTV target was higher in Bias group,and the difference of was statistically significant,but there was no significant difference in other indexes.In esophageal cancer,there were significant differences in D₂ and D_meanof GTV target,D₉₈,D₂,D_meanand TC of PTV target,and D_max of cord.But there was no significant difference in other dose parameters.In prostate cancer samples,except for TC in GTV target,the dose index in other target areas in Bias group was lower,and the difference was significant.The doses of D_mean in small intestine,V₅₀ in rectum and D_meanin left and right femur were lower,but there was no significant difference in dose indexes of other organs at risk.All targets have advantages on CI.In terms of HI,Bias group was superior in PTV target,but inferior in PGTV of brain metastasis and GTV of prostate cancer,and there was no significant difference in GTV of esophageal cancer.In Bias group,MU and DT were higher in brain metastasis of lung cancer,while MU and DT were lower in esophageal cancer,and the difference was statistically significant.In prostate cancer,the difference was not statistically significant.Conclusion:The influence of MLC leaf transmission on the plan needs to be analyzed according to the relative position of the tumor and the OARs.The organs in the target area and those without overlap with the target area are least affected by the penetration factor,and the greater the overlap with the target area,the higher the average dose;the leaf transmission affects the accuracy of TPS dose calculation,so it is necessary to maintain the machine model in TPS in time to ensure the matching degree with the machine.In the design process of sequential radiotherapy planning,the use of Bias Dose method can significantly improve the conformal degree and uniformity of the target area,improve the dose distribution of the target area,and at the same time suppress the dose of the endangered organs.It not only improves the efficiency of clinical plan design,but also simplifies the operation of plan evaluation,so it is a planning design method worth popularizing in clinic.