ABSTRACT
Objective To discuss 7Be and a 77.2 keV full-energy peak with short half-life found in the water sample from the 3D water phantom of a proton therapy system. Methods We measured the water sample from the 3D water phantom of a proton therapy system according to Determination of Radionuclides in Water by Gamma Spectrometry (GB/T 16140—2018). Results The activity concentration of 7Be in the water sample was 1.30 × 101 Bq·L−1 on December 24, 2018; 4.3 × 101 Bq·L−1 on March 22, 2019; and 1.41 × 101 Bq·L−1 at the time of sampling on December 19, 2018. On December 24, 2018, the net peak area of the 77.2 keV full-energy peak in the sample was 683 ± 45, and the measurement time was 26123.02 s; on March 22, 2019, the net peak area decreased to the background level of 194 ± 49, and the measurement time was 86400.00 s. Conclusion In the 3D water phantom of the proton therapy system, 7Be can be generated from the spallation reaction between high-energy neutrons and oxygen in water. In addition, we find a full-energy peak at 77.2 keV with short half-life. The activity concentration of 7Be in the water sample is lower than the exemption level, but the activity concentration at sampling may not be the maximum activity concentration in the process of quality control. The inductive radionuclide 7Be produced in the 3D water phantom should be identified and properly evaluated in the assessment of occupational radiation hazards of proton therapy system.
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Objective By measuring the induced radioactivity of the medical electron linear accelerator, and analyzing the measuring results and the measuring process, the cooling law of the induced radioactivity was studied, to provide basic data for evaluating additional doses to patients, medical staff and the public. Methods Five Medical Electron Linear accelerators were used to measure the induced radioactivity by using automes-6150AD6/H+6150AD-b/H x-γ radiation peripheral dose equivalent rate meter for environmental monitoring. Results The level of induced radioactivity of 5 medical electron linear accelerators is related to the factors such as measuring position and time: 10 second after the end of beam output, the maximum dose equivalent rate is 5.55 μSv/h around 5 cm on the surface of the head housing of the accelerator, and the maximum ambient dose equivalent rate at 1 m from the fixed point of the housing is 4.07 μSv/h, 5 minutes after the end of output, the maximum dose equivalent rate is 2.11 μSv/h around the 5 cm surface and 1.77 μSv/h around the 1 m fixed point. Conclusion The measured radioactivity induced by the medical electron linear accelerator gradually cools over time, the measurement valuesare maintained in a relatively fixed range, and the fluctuation range of the values is narrow.
ABSTRACT
Objective:To discuss the influence factor of induced radioactivity radiation field of high-energy medical linear accelerator and its safeguard measures.Methods: The simulation test was applied to research the radiation level of the induced radioactivity for high-energy medical linear accelerator; and to research the influences of different factors, such as radiation dose, range of exposure and site of exposure and so on, on the changes of radiation field.Results: The research showed that the high-energy X-ray radiation field strength would increase with the increasing of radiation dose, while reduce with the increasing of radiation time; and the obviously reductive trend appeared after radiated 5minutes. Meanwhile, radiation intensity would reduce with the increasing of irradiated area, but the reductive trend was not totally proportional in accordance with the increasing of irradiated area. Additionally, the head part of linear accelerator could produce the most intensity of radiation field on exposure site, and the farther away the head part of linear accelerator, the intensity the lower; and the reductive trend would increase when the distance exceeded 150cm.Conclusion: The induced radioactive radiation field intensity of high-energy medical linear accelerator has some characteristics of change. The effective safeguard measures were to reduce the work time in equipment room, reduce the adjustment time of patient position and choose reasonable exposure site and dose, and all of these measures can reduce the radiation injury.
ABSTRACT
Objective To measure and analyze Varian Clinac 21EX accelerator induced radioactivity,and to provide specific recommendations and ways of radiation protection for radiotherapy technicians.Methods To simulate the working environment of radiotherapy technician,and to detect induced radioactivity variation of Varian Clinac 21EX accelerator that induced by 15 MV X-rays under different conditions of beam field area,dose,time,distance and by high energy electron beam (12,16,20 MeV)at the different time.Results The induced radioactivity level was not influenced by different beam field area,and was increased with increasing dose (r =0.930,P ＜ 0.05),decreased with time increase (r =-0.84,P ＜ 0.05),decreased with distance increase(r =-0.975,P ＜ 0.05).The induced radioactivity attenuation levels of the different doses at the same time are different,and have the common characteristic that the induced radioactivity attenuation rate is faster in the initial times.The induced radioactivity levels of high energy electron beams were significantly lower than those of high-energy X-rays.Conclusions During radiotherapy positioning,it is necessary to take measures to protect against the induced radioactivity when high-energy rays with energy greater than 10 MeV will be used.The radiotherapy technician should take the different time and energy interval steps for the different ray type and energy and dose in order to meet the radiation protection principle of optimization.