Ionizing Radiation Standards, Institute for National Measurement Standards, National Research Council Canada, Ottawa, Ontario K1A 0R6, Canada.
Med Phys. 2010 Jan;37(1):96-107. doi: 10.1118/1.3266750.
Determine the effective point of measurement (EPOM) of 12 thimble ion chambers, including miniature chambers and three models widely used for clinical reference dosimetry. The EPOM is the point at which the measured dose would arise in the measurement medium in the absence of the probe: For cylindrical chambers, it is shifted upstream relative to the central axis of the chamber. Although current dosimetry protocols prescribe a blanket upstream EPOM shift of 0.6r, with r as the chamber cavity radius, it has been shown in recent years that the EPOM does, in fact, depend on every detail of the chamber design and on the beam characteristics. In the wake of this finding, the authors undertake a comprehensive study of the EPOM for a series of chambers in water.
This work relies on EGSnrc Monte Carlo calculations for the central axis depth dose in a water phantom and in ion chambers. They use a full Elekta Precise linac treatment head simulation to generate realistic photon beams with nominal energies of 6 and 25 MV and fields sizes of 10 x 10 and 40 x 40 cm2. The correct EPOM shift for the 12 ion chambers, modeled in realistic detail, is taken as the one minimizing the deviation of the ratio between the dose to water and the dose to the gas of the chamber cavity, according to a method proposed and validated in previous work.
The analysis reveals that the actual EPOM shift is significantly smaller than the recommended value in current dosimetry protocols, by up to 25% for reference-class chambers and 80% for miniature chambers. The location of the EPOM also depends on the characteristics of the incident beam and varies in a well-defined way with the cavity length, the central electrode radius, and the thimble wall thickness.
The authors confirm that an upstream EPOM shift of 0.6r is too large for thimble ion chambers in high energy photon beams. Proper values for the EPOM shift could be tabulated per beam and per chamber, but they envisage that a single shift for all practical beams may prove sufficient. Moreover, the systematic dependence on chamber characteristics provides evidence that a universal parametrization in terms of a few design parameters is conceivable and has implication for the calculation of chamber correction factors.
确定 12 个蘑菇型电离室的有效测量点（EPOM），包括微型室和三种广泛用于临床参考剂量学的模型。EPOM 是在没有探头的情况下测量介质中出现的测量剂量的点：对于圆柱形室，它相对于室的中心轴向前移动。尽管目前的剂量学协议规定了一个 blanket 上游 EPOM 偏移量为 0.6r，其中 r 为腔室半径，但近年来已经表明，EPOM 实际上取决于腔室设计的每个细节以及束特性。在这一发现之后，作者对一系列水腔中的 EPOM 进行了全面研究。
这项工作依赖于 EGSnrc 蒙特卡罗计算在水模体中和电离室中的中心轴深度剂量。他们使用完整的 Elekta Precise 直线加速器治疗头模拟来生成具有标称能量为 6 和 25 MV 以及 10 x 10 和 40 x 40 cm2 的射束尺寸的逼真光子束。在逼真细节中建模的 12 个电离室的正确 EPOM 偏移量被视为根据以前的工作提出并验证的方法将水腔中的剂量与腔室气体中的剂量之比的偏差最小化的偏移量。
分析表明，实际的 EPOM 偏移量比当前剂量学协议中推荐的值小得多，对于参考级室最大可达 25%，对于微型室可达 80%。EPOM 的位置还取决于入射束的特性，并随着腔长度、中心电极半径和蘑菇壁厚度以明确定义的方式变化。
作者证实，在高能光子束中，0.6r 的上游 EPOM 偏移量对于蘑菇型电离室来说太大了。可以按束和按腔列出 EPOM 偏移量的适当值，但他们设想对于所有实际束，一个单一的偏移量可能就足够了。此外，对腔室特性的系统依赖性提供了证据，表明可以根据几个设计参数进行通用参数化，并对腔室校正因子的计算具有影响。
