Ottawa Carleton Institute of Physics, Carleton University, Campus Ottawa, Ontario KIS 5B6, Canada.
Med Phys. 2009 Oct;36(10):4600-8. doi: 10.1118/1.3213094.
In the TG-21 dosimetry protocol, for cylindrical chambers in electron beams the replacement correction factor Prepl (or the product PdisPcav in the IAEA's notation), was conceptually separated into two components: the gradient correction (Pgr) accounting for the effective point of measurement and the fluence correction (Pfl) dealing with the change in the electron fluence spectrum. At the depth of maximum dose (dmax), Pgr is taken as 1. There are experimental data available at dmax for the values of Pfl (or Prepl). In the TG-51 dosimetry protocol, the calibration is at the reference depth dref=0.6R50-0.1 (cm) where Pgr is required for cylindrical chambers and Pfl is unknown and so the measured values at dmax are used with the corresponding mean electron energy at dref. Monte Carlo simulations are employed in this study to investigate the replacement correction factors for cylindrical chambers in electron beams.
Using previously established Monte Carlo calculation methods, the values of Prepl and Pfl are calculated with high statistical precision (<0.1%) for cylindrical cavities of a variety of diameters and lengths in a water phantom irradiated by various electron beams. The values of Pgr as defined in the TG-51 dosimetry protocol are also calculated.
The calculated values of the fluence correction factors Pfl are in good agreement with the measured values when the wall correction factors are taken into account for the plane-parallel chambers used in the measurements. An empirical formula for Pfl for cylindrical chambers at dref in electron beams is derived as a function of the chamber radius and the beam quality specifier R50.
The mean electron energy at depth is a good beam quality specifier for Pfl. Thus TG-51's adoption of Pfl at dmax with the same mean electron energy for use at dref is proven to be accurate. The values of Pgr for a Farmer-type chamber as defined in the TG-51 dosimetry protocol may be wrong by 0.3% for high-energy electron beams and by more than 1% for low-energy electron beams.
在 TG-21 剂量学协议中，对于圆柱形电离室在电子束中，替代校正因子 Prepl（或 IAEA 符号中的 PdisPcav），从概念上被分为两个部分：梯度校正（Pgr），用于说明有效测量点，和注量校正（Pfl），用于说明电子注量谱的变化。在最大剂量深度（dmax）处，Pgr 取为 1。在 Pfl（或 Prepl）值的 dmax 处有实验数据。在 TG-51 剂量学协议中，校准是在参考深度 dref=0.6R50-0.1（cm）处进行，此时需要圆柱形电离室的 Pgr，而 Pfl 是未知的，因此在 dmax 处测量的值与在 dref 处对应的平均电子能量一起使用。在这项研究中，采用蒙特卡罗模拟方法来研究电子束中圆柱形电离室的替代校正因子。
使用先前建立的蒙特卡罗计算方法，对于水模中各种直径和长度的圆柱形空腔，在各种电子束照射下，以高精度（<0.1%）计算了 Prepl 和 Pfl 的值。还计算了 TG-51 剂量学协议中定义的 Pgr 值。
当考虑用于测量的平行平板电离室的壁校正因子时，计算出的注量校正因子 Pfl 的值与测量值吻合较好。在电子束中，在 dref 处推导了圆柱形电离室 Pfl 的经验公式，作为腔室半径和束品质参量 R50 的函数。
在深度处的平均电子能量是 Pfl 的良好束品质参量。因此，证明了 TG-51 在 dref 处采用相同平均电子能量的 Pfl 在 dmax 处的使用是准确的。在 TG-51 剂量学协议中定义的 Farmer 型电离室的 Pgr 值对于高能电子束可能错误 0.3%，对于低能电子束可能错误 1%以上。
