Study On Key Question Of Radiation Protection For After-loading Gamma Source Brachytherapy Based On Monte Carlo

Objective:With the increasing incidence of cervical cancer,after-installed radiation therapy equipment is widely used,and the accompanying radiation protection issues have become the focus of attention.The radiation generated during radiation therapy can cause ionizing radiation hazards to the normal tissues or organs of the human body and to people around the computer room.Therefore,from the perspective of radiation therapy,radiation protection of susceptible organs and radiation protection of professionals,this study explores the radiation field generated by radioactive sources during post-installation radiation therapy,conducts research on the influencing factors of radiation dose fields in the lab room lab,and establishes the New dose estimation formula in the tract.The purpose is to more accurately analyze the radiation field generated during afterloading treatment,and promote the protection of medical ionizing radiation.Methods:Based on the Monte Carlo method,the radiation source model and the treatment room model of the retrofitted radiation therapy machine were established respectively.According to the simulation results of the radioactive source model,verify by the absorbed dose rate calculation formula,and perform MATLAB fitting,and use the origin software to obtain the radiation field of ¹⁹²Ir source in the water.Factors Affecting Dose Distribution in the Canal.At the same time,under the same conditions,the calculated value of the dose estimation formula in the national occupational health standard document（GBZ/T 201.3-2014）was compared with Monte Carlo simulation values and experimental measurements to verify its accuracy.Matlab fits the simulated data in the labyrinth to establish a more accurate formula for the dose in the labyrinth.Results:The maximum deviation between the Monte Carlo simulation results of the192Ir radioactive source and the formula calculation results was less than 2%,which proved that the radio source model established in Monte Carlo was reliable.Analysis of the radiation field of the 192Ir radiation source found that the shape of the radiation source and the surrounding medium had little effect on the radiation field.When the distance from the radiation source was greater than 0.5 cm,it could be regarded as a point source.The fitting formula of the distance value and absorbed dose rate value is Y=496.3·X^-2.227.Combining the lethal dose of the tumor and the tolerated dose of the organ,it is found that under the condition that the lethal dose of the tumor is guaranteed,the greater the distance between the reference point of the lethal dose of the tumor and the radiation source,the more vulnerable The distance between the reference point of the influencing organ dose and the reference point of the lethal dose of the tumor is greater;when the distance between the dose reference point and the radiation source is the same,the distance required by the rectum is greater than the distance required by the bladder.In addition,the maximum deviation between the simulated value of the lab room and the actual measurement result does not exceed 7.55%,which proves that the model of the lab room established in Monte Carlo is reliable.Based on Monte Carlo simulation values,the factors affecting the radiation field in the labyrinth were explored from the perspective of the decay rate and value.It was found that within a certain range,the volume change of the treatment room had no significant effect;the entrance size in the labyrinth and the distance from the radiation source to the labyrinth.The distance is proportional to the dose rate value,and the width of the labyrinth is inversely proportional;the width of the labyrinth and the distance from the source to the labyrinth are inversely proportional to the rate of decay of the dose rate,and the size of the entrance in the labyrinth is directly proportional.The relative deviation between the results of the dose estimation formula and the experimental measurement results in the national occupational health standard document（GBZ/T201.3-2014）is 46.3%to 69.0%,and the relative deviation from the Monte Carlo simulation results is 40.1%to 69.5.%between.The new formula calculates the absolute deviation of the relative dose equivalent rate in the labyrinth from the Monte Carlo simulation absolute value between 9.2%and 15.0%,which is far superior to the calculation formula in the national occupational health standard document.Conclusions:Based on the Monte Carlo method,based on the radiation field analysis of the radiation source,the absorbed dose distribution at different distances from the radiation source can be directly judged.According to the analysis of susceptible organs,the tumor and the two organs can be directly observed in the CT localized DICOM image.Distance to determine whether the susceptible organs are safe,and to help clinically develop a treatment plan.From the study of the influencing factors of the radiation dose field in the lab room of the computer room,it is known that under certain conditions,the volume change of the treatment room has little effect,and the width of the labyrinth,the entrance size within the labyrinth,and the distance of the radiation source have a great impact.The influencing factors of the dose field in the labyrinth and the new dose estimation formula provide a reference for the optimization of radiation protection shielding design in shielding design.