The Study On The Accuracy Of The Nominal Activity Of Ir192 HDR Brachytherapy Radiation Source

Brachytherapy is an important part of radiotherapy.It has some obvious advantages in the treatment of cervical cancer and prostate cancer.The high dose rate radiation source 192Ir is usually used in intracavity brachytherapy.The dose near the source is very high.With the increasing of distance,the dose decreases rapidly.The dose is approximately inverse proportional to the distance between the source and detector.Therefore,the target area receives the high dose of radiation,while the adjacent normal organs receive the lower dose.Because of the activity,location and residence time of the radioactive source determining the dose distribution of the brachytherapy,the development of the technology requires strict quality assurance and control measures.And the activity of the source is the basis of dosimetry in the brachytherapy,and the key to determining the accuracy of the whole dose calculation.Usually,the initial source activity is often calibrated in the hospital for new source activity,the nominal activity values shown by the operation system are within 3%of the acceptable range of clinical acceptability by the accurate reference value of the measured values.However,the nominal value of source activity is not verified with the decay of source activity.Therefore,different measurement methods are used to measure the source activity,and the measurement of the well type ionization chamber is taken as the reference standard.With the decay of the source,the accuracy of the nominal activity value in the control console system of the high dose rate 192Ir radiation source is studied and the related factors are analyzed.In this study,three methods were used to measure the source activity,which were recommended by the AAPM TG41 report for the well type ionization chamber measurement,and the finger type ionization chamber in water measurement method by using the finger type recommended by the AAPM TG43 report in the 1992 National Technical Supervision Bureau.Measurement of air in an ionization chamber.Using the above three methods,with a well type ionization chamber measurement results as the standard,to compare with the other two methods.In the process of source activity decay,the high dose rate Ir192 radioactive source is measured every week.The measured values of the well type ionization chamber are compared with those of the nominal activity values,the measurement results of the finger type ionization chamber in the water and the measurement results of the finger type ionization chamber in the air.The two methods of measuring the nominal activity and the finger type ionization chamber are equivalent to the measurement deviation of the well type ionization chamber,and the accuracy of the nominal activity in the decay process of the radioactive source from high to low is studied.Well type ionization chamber method using well type ionization chamber and dosimeter(HDR1000,MAX4000,standard image)to complete the measurement.In the water measurement,special jig and FC65(IBA)are used.In the air,a three-dimensional water tank is used as the stationary and walking bracket of the ionization chamber,and with the finger type ionization chamber FC65,the radiation rate at the seven locations between the ionization chamber and the source distance 10cm,15cm,20cm,25cm,30cm,35cm and 40cm are measured.In the process of measurement,the three methods are to find the position of the effective measurement points,then measure the accumulative ionization charge reading of 60s in the area.The results are applied in the calculation formula of each method,and the source activity is calculated.Then the nominal activity,the measured results in the water and the measured results in the air are compared with the measured results in the well type ionization chamber,and the results are analyzed.The absolute deviation value of the well type ionization chamber between the measurement and the nominal the average deviation of the activity is 2.363%d±0.359%,the absolute value of the deviation is 1.927%±2.823%.The results show that comparing with the results of the well type ionization chamber,the absolute value of the measured results in the air is closest to the absolute value of the well type ionization chamber,the absolute value of the deviation is 0.320%±0.189%,and the absolute deviation range is 0.040%-0.588%.The measurement deviation in water is larger than in air,with an average of 0.833%±0.270%,and the absolute value of deviation is 0.419%-1.415%.The absolute deviation of the two values was not statistically different(p=0.96±0.003).Compared with the measured results at high activity,with the decay of the source,the results of the three methods deviations have same tendency to become larger whole,especially when the source is in low activity,the trend is more obvious.But the results of three measurement methods:the P values of the independent t test of well type and nominal,in water and well type,in the air and finger were 0.96±0.003,0.955±0.003,0.935±0.007 greater than 0.05,respectively.Therefore,there is no difference in clinical statistics.That is to,the three measurement methods are all within the clinical acceptable range of less than 5%.With the using of water tank which is as the driven of ionization chamber,the source activity measured in the air is closer to the measurement results of the well type ionization chamber.The results of the three methods have high consistency and no statistical difference.Therefore,with the decreasing of source activity,the accuracy of nominal activity is getting worse.