Feasibility Study On The MRI-only Planning Of Thorax Tumors Based On Individualized Bulk Electron Density Assignment Approaches

BACKGROUND:MRI has unique advantages in radiotherapy planning design.In recent years,MRI alone radiotherapy planning?MRI-only?has been widely used in the radiotherapy of brain tumors and prostate cancer.Among them,the earliest and simplest method is to assign electron density to each target area and argans at risk?OARs?to solve the problem of lacking electron density in the design of MRI-only radiotherapy plan.However,for thorax tumors,previous studies have shown that the current commonly used electronic density assignment method is difficult to meet the clinical dose accuracy requirements.OBJECTIVE:This study was designed to investigate the dosimetric accuracy of patient-specific optimal bulk electron density?OBED?assignment approaches for simulated magnetic resonance imaging?MRI?-only planning of thorax tumors and analyze the optimal results may be achieved based on the three-dimensional dose evaluation function of the3DVH software system.METHODS:To eliminate the geometric deformation of MRI and the errors that may be introduced during CT and MRI registration,this study will use CT simulation location images to carry out individualized fine delineation of the target area and OARs of patients with thorax tumors,combined with the specific optimal bulk electron density assignment approaches,based on which to simulate the thorax tumor MRI-only plan design;The 3DVH software system was used to evaluate the dosimetric differences between the simulated MRI-only plan and the standard plan based on CT images.Patients with typical thorax tumors,including central type,peripheral type lung cancer and esophageal cancer,who received radiotherapy in our hospital were included in this study,which were immobilized by using vacuum bags and thermoplastic body models.The Philips large-aperture CT simulation positioning machine was used to acquie the CT simulation image with a scanning layer thickness and layer spacing of3mm.The resulting simulation images were transmitted to the Monaco radiotherapy treatment planning system?TPS?for target and OARs delineation,including the patient's external body contour?Body?,gross tumor volume?GTV?,adipose,muscle,bone,mediastinum,spinal cord,whole lung,lung?excluding bronchus?,bronchus,etc.The volume,average CT value and relative electron density?RED?information of the target area,tissues and organs were statistically analyzed.6MV X-rays?dosage rate600MU/min?of a Synergy accelerator was chosen for treatment planning,and a dual arc volume modulated arc therapy?VMAT?plan and a single arc dynamic conformal radiation therapy(Dynamic Conformal Arc Therap?DCAT?plan were created for each patient,the prescription dose was PTV/60Gy,2.0Gy/per fraction,5 fractions/week.After the plan optimization has been completed,the Monte Carlo algorithm was used to calculate the accurate dose distribution based on the CT electron density information of each target area,OARs and surrounding tissues.In the study,the VMAT and DCAT plans for dose calculation based on the CT image of simulation and its inherent electron density information were used as reference standard plans for subsequent comparative studies,and named as Plan-_CT.Based on the individualized fine delineation of target area and OARs of the patients with thorax tumors,and the Monte Carlo dose algorithm,the optimal electron density assignment for each target area and organs and tissues were optimized to achieve high-precision MRI-only planning,each patient will undergo VMAT/DCAT plan design and 20 dose calculations and evaluations.Specific steps:?1?Copy the standard plan Plan-_CT,and assign the average electron density value for only one target area or tissue or organ in the plan;?2?with the remaining parameters unchanged,the Monte Carlo algorithm was used to calculate the dose of the target area/tissue/organ electron density assignment plan,and a Plan-_{RED i}?i represents a certain target area or tissue,organ?was got,in which only a single target area/tissue/organ has been forced to its average electron density value plan;?3?in the comparison interface of TPS,visualization DVH comparison was conducted between Plan-_{RED i} and the reference standard Plan-_CT.If the DVH curves of each target area and organs at risk were very similar,or there were only small or even negligible differences,then 3D Gamma quantitative analysis would be conducted by using 3DVH software system,and the evaluation standard was 0.5%/±0 mm.If the obtained Gamma pass rate is greater than 95%,the average electron density assigned is considered to be the optimal electron density for the target region/tissue/organ;If the obtained Gamma pass rate is less than 95%,the assigned average electron density value would be fine-tuned,followed by a dose calculation and evaluation until the pass rate of the 3D Gamma analysis meets the experimental requirements.?4?copy the standard Plan-_CT,assign the optimal electron density values to each target area,tissue and organ,and conduct dose calculation to obtain the simulated chest tumor MRI-only Plan based on the individualized optimal electron density assignment,named as plan-_MRI;?5?Based on the 3DVH software system,the dose difference between Plan-_MRI and Plan-_CT were compared by visualizition and quantification.At this time,the criterion for the pass rate of 3D Gamma analysis was set to 1.0%±1.0mm.RESULTS:The average relative electron density values of the target areas,organs at risk and tissues were GTV 0.925,fat 0.961,muscle 1.059,bone 1.251,mediastinum 1.109,spinal cord 1.046,lung 0.286,lung?excluding bronchial?0.267 and bronchial 0.680,respectively.The standard reference plan based on CT images,Plan-_CT,compared with the simulated MRI-only optimal electron density assignment plan Plan-_MRI,under the criterion of 1.0%±1.0mm,the overall Gamma pass rate is 95.3%±1.9%,other the pass rate of each target area and OARs is between 91.9%±5.5%and 98.2%±1.9%;The maximum percentage relative deviation?MPRD?of the dosimetric parameters of gross tumor volumes?GTVs??D2,D95,D98 and Dmean?between Plan-_CT and Plan-_MRIRI was less than 0.3%,and the MPRD of the average dose for OARs was less than0.5%,and the MPRD of the average values for lung V5,V20,V30 and V40 were less than 0.2%.CONCLUSIONS:For thorax tumors,the simulated MRI-only plan obtained can achieve the dose accuracy similar to the standard reference plan based on the fine delineation of target area and OARs and the OBED assignment method.Studies have shown that this method is an effective method for MRI-only radiotherapy planning for thorax tumors.