Chow James C L, Grigorov Grigor N
Radiation Medicine Program, Princess Margaret Hospital and Department of Radiation Oncology, University of Toronto, Toronto, Ontario, Canada.
Department of Physics, University of Waterloo, Waterloo, Ontario, Canada.
J Appl Clin Med Phys. 2009 Apr 29;10(2):75-91. doi: 10.1120/jacmp.v10i2.2918.
This study investigates the dosimetric dependence of the dimension of a lead (Pb) layer for shielding using clinical electron beams with different energies. Monte Carlo simulations were used to generate phase space files of the 4, 9 and 16 MeV electron beams produced by a Varian 21 EX linear accelerator using the EGSnrc-based BEAMnrc code, and validated by measurements using films. Pb layers with different thicknesses (2, 4, 6 and 8 mm) and diameters (2.5, 3, 3.5 and 4 cm) were placed at the center of an electron field on a Solid Water phantom. Beam profiles were determined at the depth of maximum dose (d(m)) using Monte Carlo simulations. The line doses under the Pb layer at d(m) including the penumbra at the edge of the layer and relative dose at the central beam axis (CAX) were studied with varying thickness and diameter of the layer. It is found that 2 mm of Pb layer is adequate to provide 5 half value layer (HVL) attenuation for the 4 MeV electron beams, and the beam profiles at dm under the Pb layer depend on the diameter but the thickness of the Pb. However, for the 9 and 16 MeV electron beams, the relative dose at the CAX and dm depends on both the thickness and diameter of the Pb layer. For 8 mm thickness of Pb, 4 and 5 HVL attenuation of electron beams with energies of 9 and 16 MeV can be achieved at d(m), respectively. Moreover, the beam profile under the Pb layer at dm depends on (1) the penumbra region at the edge of the Pb layer, (2) the beam attenuation varying with the thickness of the Pb layer, (3) the electron side scatter contributing to the CAX under the Pb layer, and (4) the photon contamination produced by the Pb layer. A parameter called "shielding area factor", defined as the ratio of the length between two points of 50% relative doses in the beam profile at dm to the diameter of the Pb layer, is suggested to the radiation oncology staff to predict the required size and thickness of Pb for shielding a target with known dimension at d(m). The dosimetric data calculated by Monte Carlo simulations in this study is useful to select what thickness and size of Pb are suitable for the protection of critical tissue in electron radiotherapy.
本研究调查了使用不同能量的临床电子束时，用于屏蔽的铅（Pb）层尺寸的剂量学依赖性。使用基于EGSnrc的BEAMnrc代码，通过蒙特卡罗模拟生成瓦里安21EX直线加速器产生的4、9和16 MeV电子束的相空间文件，并通过使用胶片的测量进行验证。将不同厚度（2、4、6和8 mm）和直径（2.5、3、3.5和4 cm）的Pb层放置在固体水模体上电子射野的中心。使用蒙特卡罗模拟在最大剂量深度（d(m)）处确定射野轮廓。研究了在d(m)处Pb层下方包括层边缘半影和中心射束轴（CAX）处相对剂量在内的线剂量随层厚度和直径的变化。结果发现，2 mm的Pb层足以对4 MeV电子束提供5个半价层（HVL）的衰减，且Pb层下方d(m)处的射野轮廓取决于直径而非Pb的厚度。然而，对于9和16 MeV电子束，CAX和d(m)处的相对剂量取决于Pb层的厚度和直径。对于8 mm厚的Pb，在d(m)处分别可实现9和16 MeV能量电子束4和5个HVL的衰减。此外，Pb层下方d(m)处的射野轮廓取决于：（1）Pb层边缘的半影区域；（2）随Pb层厚度变化的射束衰减；（3）对Pb层下方CAX有贡献的电子侧向散射；（4）Pb层产生的光子污染。建议向放射肿瘤学工作人员引入一个名为“屏蔽面积因子”的参数，该参数定义为d(m)处射野轮廓中两个50%相对剂量点之间的长度与Pb层直径的比值，以预测为在d(m)处屏蔽已知尺寸的靶区所需的Pb尺寸和厚度。本研究中通过蒙特卡罗模拟计算得到的剂量学数据，对于选择适合电子放射治疗中保护关键组织的Pb厚度和尺寸很有用。
