中外近距离治疗机房辐射屏蔽设计标准应用比对

Comparison of Chinese and international radiation shielding standards in application for after loading bunker shielding design

目的中外近距离治疗机房辐射屏蔽设计考虑因素不尽相同,本研究以常见的高剂量率192Ir源为例,分别应用国内外标准进行后装机房的屏蔽核算,比较计算结果分析差异产生的原因,为修订和完善现行国家标准提供参考。方法对于典型的后装机房进行工作量估算,放射源初始活度10 Ci(1 Ci=3.7×10^(10) Bq),分别按照英国医学物理与工程研究所IPEM75号报告、美国辐射防护委员会NCRP151报告和GBZ/T 201.3-2014国家标准设计后装机房屏蔽方案,详细比较不同参考标准的屏蔽限值、居留因子及其他因子的差异。结果典型后装机房的年照射时长约为330 h,按照NCRP151报告、IPEM法规和GBZ/T 201.3-2014国家标准计算得到的控制室、屏蔽墙外、候诊区、相邻控制室和无人居留室顶等关注点位所需的混凝土厚度分别为70、65、61、70、50 cm,41、43、30、40、39 cm和84、79、46、88、39 cm。按照GBZ/T 201.3-2014国家标准计算得到的相应关注点所需的混凝土屏蔽厚度普遍偏厚,与NCRP151报告结果差别较小,IPEM75号报告计算得到的屏蔽厚度最薄;三者计算出的防护门的等效铅屏蔽厚度分别为1.170、0.854和1.040 cm,厚度相近。结论我国现行后装机房屏蔽标准所推荐的计算方法和评价指标计算得到的屏蔽厚度与NCRP151报告的相似但偏保守,特别是现行国家推荐标中要求的瞬时剂量当量率评价指标以及过于保守的居留因子取值会显著增加主屏蔽区所需的屏蔽厚度。

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.