Research On Absolute Measurement And Delivery Method Of Absorbed Dose To Water In Medical Electron Beams

Medical electron beam is an important component of modern radiotherapy technology.Because of its limited range,the radiation dose is deposited in a relatively shallow range.It is mainly used for the treatment of superficial or eccentric tumors,which can effectively protect the tissues and organs behind the tumor.As the prescription dose of radiotherapy,the absorbed dose to water is also the key physical quantity for calibration of radiotherapy dose in clinical electron beams.At present,the cause of most radiotherapy accidents is the transmission error of absorbed dose to water during treatment.Through the research on absolute measurement and delivery method of absorbed dose to water in medical electron beams,the uncertainty is directly reduced from the root,so as to reduce the radiotherapy accidents caused by the error of radiotherapy dose delivery and improve the effect of radiotherapy.The main research contents and results of the thesis are as follows:In order to reduce the measurement uncertainty of absorbed dose to water in medical electron beams,this paper uses the calorimetry to directly measure the temperature rise of 0.5 m K with an uncertainty of 0.15%,which is generated by the deposited energy of the electron beams in the water,and then studies the influence factors in absolute measurement of absorbed dose to water.The model of absolute measurement of absorbed dose to water is established.The uncertainty of absolute measurement of absorbed dose to water in electron beams is reduced to 0.35%,which is lower than the current international absolute measurement standard of 0.4%.By analyzing the calibration method of absorbed dose to water in clinical,the delivery method of absorbed dose to water with uncertainty as low as 1.05%is given,which meets the requirement of calibration uncertainty of absorbed dose to water less than 2%in clinical electron beams.In order to solve the problem of low temperature rise and high measurement accuracy of medical electron beam irradiation,an electron beam water calorimeter is designed and built,which is composed of two thermistor probes with a sensitivity of about 4.6%/K,a quasi adiabatic water phantom with a temperature drift as low as 1.6μK/s and an AC bridge with a resolution as low as 10 n V.In the measurement of radiation temperature rise distribution,it is found that the dose gradient is relatively small after the maximum dose of electron beam with energy more than 12 Me V,which is more suitable for direct measurement of absorbed dose to water by water calorimeter.The test results of AC Bridge show that when the excitation voltage is 0.5 V,the frequency is 19.5 Hz,and the resistance accuracy is 0.01%,where the system noise is the smallest.When the lock-in amplifier range is 20μV,the measurement uncertainty of the radiation temperature rise could be 0.15%.The influence factors of absolute measurement of absorbed dose to water in electron beam measured are studied.By setting a convection barrier,electromagnetic stirring and when the water temperature is 4°C,the influence of heat convection that may be generated during temperature rise measurement is effectively eliminated.COMSOL was used to construct the temperature environment of the water phantom,and the influence of heat conduction generated by the calorimeter glass vessel and dose distribution was calculated.By introducing a glass vessel,the organic impurities in the water around the thermistor probes are reduced to 2.5×10^–7 mol L^–1,and filling H₂ to inhibit the yield of radiation chemical reaction,so that the effect of radiation chemical reaction is reduced to 0.15%.According to the density change caused by the water temperature change and the electron beam depth dose distribution,the correction amount of the water density change is calculated.Used the PTW 31015 ionization chamber to measure the disturbance correction of the glass vessel to the radiation field.By discarding the signal 20s after the end of irradiation,the influence of the thermistor’s time response is eliminated.The model of absolute measurement of absorbed dose to water in medical electron beams is established.The absolute measurement and uncertainty evaluation of absorbed dose to water in10 Me V,12 Me V,15 Me V,18 Me V,20 Me V and 22 Me V electron beams have been completed.According to the model of absolute measurement of absorbed dose to water in medical electron beams,the uncertainty components of each influence factor are analyzed,and according to the variance composition theorem,the integrated standard uncertainties of absorbed dose to water in electron beams is 0.34%,0.32%,0.32%,0.36%,0.33%and 0.32%respectively.A delivery method of giving the calibration coefficient of absorbed dose to water for the ion chamber with R₅₀ of 7.5 cm beam is proposed,and the electron beam energy conversion factors are used to reduce the uncertainty of the calibration in clinical electron beams with a parallel-plate ion chamber from 2.1%to 1.05%.Five types of 9ionization chambers were used to measure the beam quality conversion factors,and some calibration recommendations for absorbed dose to water in clinical electron beams were given.The results show that the beam quality conversion factor of the ionization chamber and its division into photoelectric conversion factor and energy conversion factor can all be used as constants.