Estimation of the cable effect in megavoltage photon beam by measurement and Monte Carlo simulation
Estimation of the cable effect in megavoltage photon beam by measurement and Monte Carlo simulation
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通过测量和蒙特卡罗模拟估计兆伏光子束中的电缆效应
DOI：
10.1002/mp.14450
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发表时间：
2020
期刊：
影响因子：
3.8
通讯作者：
Saitoh Hidetoshi
中科院分区：
文献类型：
--
作者：
Yamauchi Ryohei;Igari Mitsunobu;Kasai Yuya;Hariu Masatsugu;Suda Yuhi;Kawachi Toru;Katayose Tetsurou;Mizuno Norifumi;Miyasaka Ryohei;Saitoh Hidetoshi
目的电离室被广泛用于大电压光子束的剂量测量。电离室的几个特性，包括电缆效应、极性效应和离子复合损耗，在标准剂量学协议中都有描述。电缆效应被归类为泄漏电流和康普顿电流，不仅在参考剂量学中，而且在大范围内也描述了对这些因素的仔细考虑。然而，康普顿电流在电缆中的产生机理还没有得到深入的研究。用蒙特卡罗模拟方法研究了6 mV X射线束中电离室的电缆效应，并用蒙特卡罗模拟方法研究了康普顿电流产生的机理。材料与方法用4个相同型号、不同电离体积的PTW电离室(TM30013、TM31010、TM31014和TM31016)测量输出因数(OPF)和电缆效应。测量OPF以观察电缆效应引起的任何变化。分别用泄漏电流和康普顿电流来评估电缆效应，并用一种新的方法估计每电缆长度的每吸收剂量对水的电荷量。分析了芯线中电子和正电子的行为，并通过蒙特卡罗模拟计算了光子束的康普顿电流。结果在OPF测量中，小室和微室的静电计读数在极性上的差异变得明显，而电离体积越小，差异越大。对于电缆效应的测量，确定了泄漏电流对电缆效应的贡献可以忽略不计，而康普顿电流占主导地位。对于pTW电离室，每条电缆长度的每吸收剂量的康普顿电流所产生的电荷估计为0.36±0.03pc Gy1 cm−1−1。结果，康普顿电流对静电计读数的贡献估计分别为0.002 cm−1(农夫类型)、0.011%cm−1(扫描)和0.088%cm−1(微室)。模拟结果表明，MV X射线产生的康普顿电流不仅可以用康普顿散射产生的反冲电子来解释，也可以用正电子对的产生来解释。由流出电荷和流入电荷的差值估算出的康普顿电流为0.45pCy−1 cm−1，与实测值相当。结论通过测量，定量地估计了几个腔室的电缆效应，并用蒙特卡罗模拟方法对康普顿电流的产生机制进行了探讨。已确定康普顿电流是电缆效应的主要成分，其电荷始终为正且几乎相同，与电离室的体积无关。对于电离室，估计了康普顿电流对静电计读数的贡献。分析了康普顿电流产生的机理，证实了康普顿电流可以通过芯线的流出电荷和流入电荷的差值来估算。
PurposeIonization chambers are widely used for dosimetry with megavoltage photon beams. Several properties of ionization chambers, including the cable effect, polarity effect, and ion recombination loss, are described in standard dosimetry protocols. The cable effect is categorized as the leakage current and Compton current, and careful consideration of these factors has been described not only in reference dosimetry but also in large fields. However, the mechanism of Compton current in the cable has not been investigated thoroughly. The cable effect of ionization chambers in 6 MV X‐ray beam was evaluated by measurement, and the mechanism of Compton current was investigated by Monte Carlo simulation.Materials and MethodsFour PTW ionization chambers (TM30013, TM31010, TM31014, and TM31016) with the same type of mounted cable, but different ionization volumes, were used to measure output factor (OPF) and cable effect measurement. The OPF was measured to observe any variation resulting from the cable effect. The cable effect was evaluated separately for the leakage current and Compton current, and its charge per absorbed dose to water per cable length was estimated by a newly proposed method. The behavior of electrons and positrons in the core wire was analyzed and the Compton current for the photon beam was estimated by Monte Carlo simulation.ResultsIn OPF measurement, the difference in the electrometer readings by polarity became obvious for the mini‐ or microchamber and its difference tended to be larger for a chamber with a smaller ionization volume. For the cable effect measurement, it was determined that the contribution of the leakage current to the cable effect was ignorable, while the Compton current was dominant. The charge due to the Compton current per absorbed dose to water per cable length was estimated to be 0.36 ± 0.03 pC Gy−1cm−1for PTW ionization chambers. As a result, the contribution of the Compton current to the electrometer readings was estimated to be 0.002% cm−1for the Farmer‐type, 0.011% cm−1for the scanning, and 0.088% cm−1for microchambers, respectively. By the simulation, it was determined that the Compton current for MV x‐ray could be explained by not only recoil electrons due to Compton scattering but also positron due to pair production. The Compton current estimated by the difference in outflowing and inflowing charge was 0.45 pC Gy−1cm−1and was comparable with the measured value.ConclusionThe cable effect, which includes the leakage current and Compton current, was quantitatively estimated for several chambers from measurements, and the mechanism of Compton current was investigated by Monte Carlo simulation. It was determined that the Compton current is a dominant component of the cable effect and its charge is consistently positive and nearly the same, irrespective of the ionization chamber volume. The contribution of Compton current to the electrometer readings was estimated for chambers. The mechanism of Compton current was analyzed and it was confirmed that the Compton current can be estimated from the difference in outflowing and inflowing charge to and from the core wire.