Research On Quality Control And Assurance Technology Of Afterloading Radiation Therapy

High Dose Rate（HDR）brachytherapy allows for a very high dose to the tumor with relative sparing of the surrounding normal structures.The emergence of afterloading devices has greatly reduced the occupational exposure dose of medical staff,making brachytherapy widely used.In the past 20 years,HDR afterloading radiotherapy has become an important means of treating cancer,which is widely used in cervical cancer and other cancers.The half life of Ir-192 is 73.83 days,requiring3-4 times source replacement each year to maintain short treatment times.The new-replaced source need to be calibrated before used for clinical treatment.Once the dose deviation occurs during the treatment,it will cause great damage to the surrounding normal tissues,owing to the dose of single afterloading radiotherapy is high.Therefore,the quality control and quality assurance of afterloading radiotherapy are of clinical importance.This paper studies the quality control and quality assurance of afterloading radiotherapy.The main research contents are as follows:（1）Compare their differences of the two methods of the sources activity calibration,and discuss the applicability in clinical practice.Three MicroSelectron Ir-192 sources were measured using two different source activity calibration methods.The first method uses the setting provided by the hygienic industry standard（WS262-2017）to measure the charge at the maximum sensitive position of the well type ionization chamber at 300Vbias,using the formula provided in WS262-2017 to calculate the source activity.The second method uses the setting provided by the well type ionization chamber manufacturer to measure the charge at the maximum sensitive position of the well type ionization chamber at 400V bias,and calculates the source activity using the formula provided by the manufacturer.Compare the deviation between the measured results and the theoretical calculated values.According to the method provided by WS262-2017 and the manufacturer,the activity of the radioactive sources is A_W and A_P,respectively.The minimum deviation between A_W and A_P is 0.86%,and the maximum deviation is 3.02%.Compared with the theoretical calculation A₀,the minimum deviation between A_W and A₁ is-0.44%,the maximum deviation is 2.59%;the minimum deviation between A_P and A₁ is 1.02%,the maximum deviation is 4.13%.The results measured according to WS262-2017and the PTW method are in good agreement.（2）To explore the feasibility of finger-type ionization chambers for point-dose verification of HDR post-loading radiotherapy plans.In this paper,a reasonable experimental phantom was designed to simulate afterloading radiotherapy for the treatment of radical cervical cancer.And we has designed simple plan and 21 clinical validation plans to verificate the point dose with the finger-type ionization chamber.The deviation between planning system calculated value and the measured value is analyzed.From the results,the maximum deviation between the two is 3.57%,the rest are less than 3%,which meet the industry standard that the dose deviation must be less than 5%.It is indicated that the figure-type ionization chamber can be used for point dose verification of afterloading radiotherapy plan.In order to study the influence of the applicator reconstruction bias or the applicator shift on the planning system value,the reconstruction of the applicator in the planning system was changed in this paper.The results showed that in the case of the same amount of change,the maximum deviation of reconstruction in the X-axis direction,Y-axis direction,Z-axis direction is 0.6%,1.5%,8.6%,respectely.Therefore,during the experiment or in clinical treatment,the movement of the phantom or patient should be reduced to avoid increasing the uncertainty of the verification.