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Objective:To verify radiation shielding effect of the maze of the CyberKnife room, so as to identify and correct the deficiencies in the shielding designs in certain circumstances.Methods:In line with the radiation protection shielding design scheme of the CyberKnife room provided by the producer, the consideradtions are focused on the useful beam that could mainly pass through the image center, but not the outer wall of the maze. However, during the inspection and acceptance of radiological protection in the built room, it was found that in a certain situations useful wire did not pass through the imaging center. Therefore, the additional shielding and protection facilities were built and verified.Results:After verification and acceptance on the scene, in the case where the useful beam was passing through the image center, the highest ambient dose equivalent rate at the concerned points 30 cm away from the outer wall of the maze was 0.31 μSv/h, less than the control level 10 μSv/h. In the opposite case, the highest ambient dose equivalent rate at the same points as above was 301.67 μSv/h, ablut 30 times as much as the control level. After the thickness in maze outer wall was enhanced, the highest ambient dose equivalent rate at the same as above was 2.14 μSv/h. This testing result met the concerned national standard.Conclusions:It is desirable that in designing the outer wall shielding in the maze in a CyberKnife room, attention should be paid to whether or not the useful beam could pass through the image center, or otherwise directly irradiate on the maze on the basis of the movement range of the accelerator. Shielding wall thickness should be calculated on the basis of the irradiation ragne and the distance between source location and the concerned points to ensure being in compliance with the requirements of the concerned national standard. Meanwhile, it should protect the workers occupying at the concerned locations from receiving higher radiation doses.
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Objective:To assess the shielding requirements of low energy X-ray intraoperative radiotherapy room under the domestic and foreign standards and guidelines, to measure the sured transmission factors for a range of shielding materials, the ambient dose equivalent rate around concerned positions and the shielding effect of protective devices, so as to provide references for shielding design of such radiotherapy units and applications of radiological protection devices.Methods:The required shielding thicknesses for a treatment room with INTRABEAM intraoperative radiotherapy system were calculated under the Chinese national standard GBZ 121, IPEM report No. 75 and NCRP report No. 151, respectively. The transmission factors for a range of shielding materials including solid water slab, shielding sheet and shielding apron were measured. Moreover, the ambient dose equivalent rates were measured under the simulated working conditions and the shielding effectiveness of a lead screen was evaluated.Results:The required lead thicknesses calculated under different standards and guidelines were less than 0.6 mm for all the concerned points, with the differences at sub-millimeter level. The low energy X-rays generated by this equipment attenuated rapidly in the shielding materials. The measured transmission factors of 0.05 mm lead equivalent shielding sheet and 0.25 mm lead equivalent shielding apron were 0.068 and 0.003 8, respectively. When the radiation was delivered using spherical applicator without any attenuation, the ambient dose equivalent rates at 1 m and 2 m from the X-ray source were 10.7 and 2.6 mSv/h, respectively. The corresponding measurement values decreased to 3.8 and 0.9 μSv/h, respectively, when the spherical applicator was inserted into a small water tank. Meanwhile, the ambient dose equivalent rate at 2 m was reduced to the background level when using protective screen.Conclusions:The shielding requirements for a low energy X-ray intraoperative radiotherapy facility are minimal, with low effective energy of X-rays generated by this equipment, but the dose rate close to the unshielded radiation source is high. The shielding scheme of treatment room should be optimized in design and the protective device should be used in a reasonable way.
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Objective To analyze the room layout and shielding of three types of self-shielded accelerators, and to provide a basis for optimizing the shielding design of rooms for self-shielded accelerators. Methods We compared the radiation shielding levels (transmitted radiation dose rate outside the primary barrier and scattered radiation dose rate outside the secondary barrier) of three self-shielded accelerator rooms by Monte Carlo simulation and empirical formula calculation. Results According to Monte Carlo simulation and empirical formula calculation, for the Unity MR Linac accelerator, the scatter radiation dose rate outside the secondary barrier was significantly higher (up to five times) than the transmitted primary radiation dose rate outside the primary barrier. The scatter radiation dose rate of the cross section was significantly higher than that of the sagittal section for both Unity MR Linac and TOMO accelerators. Conclusion The differences in the shape, material, and thickness of self-shielding structures complicate the shielding calculation and design for accelerator rooms. The shielding calculation method should be improved to optimize the radiation shielding of novel accelerator rooms.
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Objective:To compare the calculation result and analyzes the reasons for their differences so as to provide reference for the revision and improvement of the current national standards on radiation shielding design for the room of brachytherapy.Methods:For the initial activity 10 Ci (1 Ci=3.7×10 10 Bq) of radioactive sources, the shielding schemes of brachytherapy room were designed in accordance with UK Institnte of Physics and Engineering in Medicine(IPEM) Report 75, USA NCRP Report 151 and the national standard GBZ/T 201.3-2014, respectively. The differences in shielding limits, occupancy factors and other relevant factors are compared in detail. Results:The annual exposure time in a typical brachytherpy room was about 330 h. The point-specific concrete thickness were 70, 65, 61, 70, 50 cm as required by NCRP Report 151, 41, 43, 30, 40, 39 cm by IREM regulations and 84, 79, 46, 88, 39 cm by GBZ/T 201.3, respectively. The concerned concrete shielding thickness calculated under the GBZ/T 201.3-2014 was generally thicker, with lesser difference from NCRP Report 151 result, whereas that from the IPEM75 report was thinnest. The equivalent lead shielding thicknesses of the protective doors calculated using the three method are 1.170, 0.854 and 1.040 cm, respectively.Conclusions:The shielding thickness calculated using the calculation method and evaluation index recommended by the current Chinese shielding standards for brachytherapy bunker is similar to that reported in NCRP151, but is conservative. In particular, the evaluation index of instantaneous dose equivalent rate required by the current national standards and the relative conservative value of occupancy factor will significantly increase the shielding thickness required by the main shielding area.