Research On X-Ray Dose Detector Based On Scintillating Material For Medical Applications

In recent years,the number of people suffering from cancer in the world has increased year by year,and radiotherapy has been widely concerned as one of the three effective means of treating cancer.The ideal goal of radiotherapy is to minimize the impact on surrounding healthy tissue while ensuring adequate doses in the target area of the lesion.Radiation therapy has been developed to this target for decades:from three dimensional conformal radiotherapy(3D-CRT),intensity-modulated radiotherapy(IMRT),image-guided radiotherapy(IGRT),to adaptive radiotherapy(ART).Although the new radiotherapy technology is developing rapidly,the efficacy of radiotherapy is still can not meet the ideal requirements.Up to now,three steps of preoperative planning,radiation therapy,and post-surgery are still used in radiation therapy.Before radiotherapy,the human body is irradiated and modeled by CT,and the patient's treatment plan is determined through simulation calculation.In actual treatment,the movement of the human body and the tumor tissue will have a considerable impact on the rationality of the plan,even if subsequent verification of the dose does not compensate for the damage caused during the treatment.If there is a radiation dose detecting device capable of detecting the real-time absorbed dose of the tumor and its surrounding tissues in real time during the operation,the treatment plan can be adjusted in real time during the treatment process,thereby truly meeting the demand for "precision radiotherapy".In order to fill the technical gap of real-time dose monitoring during radiotherapy and make "precision radiotherapy" become a reality in the near future,this thesis has innovatively designed and developed an embedded optical fiber sensor structure,and developed a radiation dose detection system based on this design scheme.The dose detection system can detect the X-ray radiation dose of tumor patients receiving radiation therapy in real time,thus realizing immediate feedback on the accelerator output dose,making real-time adjustment of the radiotherapy plan and "precision radiotherapy" possible.The main contents of this thesis include:Based on the analysis of the existing inorganic scintillating X-ray dose detection probes,the emission spectra of various inorganic scintillation materials commonly used under X-ray irradiation are tested to select a perfect material which has high light output intensity,high dose rate linear material and suitable for PMMA fiber transmission.A new structure of inorganic scintillation X-ray dose detection probe was independently designed using PMMA fiber:an embedded structure probe.And the optical transmission characteristics of the structure were tested.The design advantages of the new structure were verified by both theory and experiment.Compared with the traditional design scheme,the response strength of the embedded probe can theoretically increase by more than 30 times.Considered with the actual needs of radiotherapy,the machine of the medical X-ray detection system was completed.According to the clinical needs of radiotherapy and international standards,the characteristics of the medical X-ray detection system repeatability,dose linearity,angle dependence,and percent depth dose distribution curve were measured.A detailed study on the characteristics of inorganic scintillation materials without water equivalence has been carried out,and new speculations which is different from the equivalent atomic number and the Cerenkov radiation are proposed and verified.The prototype medical X-ray detection system was verified and awarded at the National Institute of Metrology twice.The results show that the repeatability relative error of the fiber X-ray detection system is only 0.1%;the linear correlation coefficient R2 of the dose linearity is greater than 0.9999,which shows extremely high dose linearity;the maximum relative error is only 0.35%under different radial angle illumination that can be considered as angle-independent;the leakage after irradiation is less than 0.005%,which is far lower than the design standard of the ionization chamber as the"gold standard" in the dose measurement field.Based on the test results of the air ionization chamber,a response intensity-dose relationship experiment at different depth was designed to complete the determination of the X-ray quality correction factor under the medical accelerator.The difference between the percent depth dose and off-axis ratio curves is corrected by mathematical calibration and physical structure optimization.For the first time,the effect of non-water equivalence of materials in radiation dosimetry was solved.The results of the authoritative certification show that the fiber X-ray dose detection system has a high application potential,providing a very important feasibility solution for real-time adjustment and "precision radiotherapy" during the radiotherapy planning process.