Seuntjens J, Verhaegen F
Institute for National Measurement Standards, Ionizing Radiation, Standards, National Research Council, Ottawa, Canada.
Med Phys. 1996 Oct;23(10):1789-96. doi: 10.1118/1.597833.
In this paper we examine the depth and field size dependence of the overall correction factor kch for in-phantom dose determinations in orthovoltage x-ray beams. The overall correction factor is considered to be composed of three contributions, i.e., (1) a contribution from the angular dependence of the chamber response free-in-air, derived based on the measured directional response of the NE2571 for different energies combined with Monte Carlo calculations; (2) a displacement effect and (3) a stem effect, both calculated using the Monte Carlo method for different field sizes and depths. The results show a variation of, at most, 2.2% at the lowest photon energies (29.8-keV average photon energy) when going from 2 cm to 5 cm for a small circular 20-cm2 field. In the medium-energy range (> or = 100 kV), variations are limited to, at most, 1.5% for 120 kV-150 kV when comparing the most extreme variations in field size and depth (i.e., 2-cm depth; 20-cm2 area compared to 5 cm depth; 200-cm2 area). Depth variations most importantly affect the overall correction factor by hardening of the photon fluence spectrum, whereas field diameter variations affect the factor by increase or decrease of contributions of photon scattering. The work shows that taking into account the uncertainties adopted in the recent review of data and methods recommended in the IAEA code of practice, the dependence of the overall correction factor on depth and field size is insignificant for the radiation qualities between 100 kV (HVL 0.17-mm Cu, average energy: 52 keV) and 280 kV (HVL 3.41-mm Cu, average energy: 144 keV).
在本文中，我们研究了在正交电压X射线束中进行体模内剂量测定时，总体校正因子kch对深度和射野大小的依赖性。总体校正因子被认为由三部分组成，即：(1)基于NE2571在不同能量下的测量方向响应结合蒙特卡罗计算得出的电离室在空气中响应的角度依赖性贡献；(2)位移效应；(3)茎效应，这两者均使用蒙特卡罗方法针对不同射野大小和深度进行计算。结果表明，对于一个20平方厘米的小圆形射野，在最低光子能量（平均光子能量29.8千电子伏）下，从2厘米深度到5厘米深度时，变化最多为2.2%。在中能范围（≥100千伏），当比较射野大小和深度的最极端变化（即2厘米深度；20平方厘米面积与5厘米深度；200平方厘米面积）时，120千伏至150千伏的变化最多限制在1.5%。深度变化最重要的是通过光子注量谱的硬化来影响总体校正因子，而射野直径变化则通过光子散射贡献的增加或减少来影响该因子。这项工作表明，考虑到国际原子能机构实践准则中近期数据审查和方法推荐所采用的不确定性，对于100千伏（半值层0.17毫米铜，平均能量：52千电子伏）至280千伏（半值层3.41毫米铜，平均能量：144千电子伏）之间的辐射质量，总体校正因子对深度和射野大小的依赖性不显著。
