Ottawa Carleton Institute for Physics, Carleton University Campus, 1125 Colonel By Drive, Ottawa, Ontario K1S 5B6, Canada.
Med Phys. 2011 Feb;38(2):1081-8. doi: 10.1118/1.3532818.
Recent Monte Carlo calculations of beam quality conversion factors for ion chambers that use high-Z electrodes [B. R. Muir and D. W. O. Rogers, Med. Phys. 37, 5939-5950 (2010)] have shown large deviations of kQ values from values calculated using the same techniques as the TG-51 and TRS-398 protocols. This report investigates the central electrode correction factor, Pcel, for these chambers.
Ionization chambers are modeled and Pcel is calculated using the EGSnrc user code egs_chamber for three cases: in photon and electron beams under reference conditions; as a function of distance from an iridium-192 point source in a water phantom; and as a function of depth in a water phantom on which a 200 kVp x-ray source or 6 MV beam is incident.
In photon beams, differences of up to 3% between Pcel calculations for a chamber with a high-Z electrode and those used by TG-51 for a 1 mm diameter aluminum electrode are observed. The central electrode correction factor for a given value of the beam quality specifier is different depending on the amount of filtration of the photon beam. However, in an unfiltered 6 MV beam, Pcel, varies by only 0.3% for a chamber with a high-Z electrode as the depth is varied from 1 to 20 cm in water. The difference between Pcel calculations for chambers with high-Z electrodes and TG-51 values for a chamber with an aluminum electrode is up to 0.45% in electron beams. The central electrode correction, which is roughly proportional to the chambers absorbed dose sensitivity, is found to be large and variable as a function of distance for chambers with high-Z and aluminum electrodes in low-energy photon fields.
In this work, ionization chambers that employ high-Z electrodes have been shown to be problematic in various situations. For beam quality conversion factors, the ratio of Pcel in a beam quality Q to that in a Co-60 beam is required; for some chambers, kQ is significantly different from current dosimetry protocol values because of central electrode effects. It would be best for manufacturers to avoid producing ion chambers that use high-Z electrodes.
最近使用高 Z 电极的离子室的束质量转换因子的蒙特卡罗计算[B. R. Muir 和 D. W. O. Rogers，医学物理学 37,5939-5950（2010）]表明，kQ 值与 TG-51 和 TRS-398 协议中使用的相同技术计算的值存在很大偏差。本报告研究了这些腔室的中心电极校正因子 Pcel。
使用 EGSnrc 用户代码 egs_chamber 对三种情况进行建模并计算 Pcel：在参考条件下的光子和电子束中；在水模体中铱-192 点源的距离处；以及在水模体上入射 200 kVp X 射线源或 6 MV 束的深度处。
在光子束中，观察到具有高 Z 电极的腔室的 Pcel 计算值与 TG-51 用于直径 1 毫米的铝电极的值之间的差异高达 3%。给定束质量指示符的值的中心电极校正因子取决于光子束的过滤量。然而，在未过滤的 6 MV 束中，当水深处从 1 到 20 cm 变化时，具有高 Z 电极的腔室的 Pcel 仅变化 0.3%。具有高 Z 电极的腔室的 Pcel 计算值与 TG-51 用于铝电极的腔室的计算值之间的差异在电子束中高达 0.45%。中心电极校正因子大致与腔室的吸收剂量灵敏度成正比，在低能光子场中，具有高 Z 和铝电极的腔室的距离变化很大且变化很大。
在这项工作中，已经表明在各种情况下使用高 Z 电极的离子室存在问题。对于束质量转换因子，需要束质量 Q 中的 Pcel 与 Co-60 束中的 Pcel 之比；对于某些腔室，由于中心电极效应，kQ 与当前剂量学协议值显着不同。制造商最好避免生产使用高 Z 电极的离子室。
