Acoustics and Ionising Radiation Team, National Physical Laboratory, Hampton Road, Teddington TW11 0LW, United Kingdom.
Med Phys. 2010 Jun;37(6):2876-89. doi: 10.1118/1.3427411.
Ion recombination for ionization chambers in pulsed high-energy photon beams is a well-studied phenomenon. Despite this, the correction for ion recombination is often determined inaccurately due to the inappropriate combination of using a high polarizing voltage and the simple two-voltage method. An additional complication arises in new treatment modalities such as IMRT and tomotherapy, where the dosimetry of a superposition of many constituting fields becomes more relevant than of single static fields. For these treatment modalities, the irradiation of the ion chamber geometry can be instantaneously inhomogeneous and time dependent.
This article presents a study of ion recombination in ionization chambers used for dosimetry in a helical tomotherapy beam. Models are presented for studying the recombination correction factors in a continuous beam, in pulsed large and small fields, and in helical fields. Measurements using Exradin A1SL, NE2571, and NE2611 type chambers and Monte Carlo simulations using PENELOPE are performed in support of these models.
Initial recombination and charge multiplication are found to be the same in 60Co and in the pulsed high-energy photon beam for the chambers and operating voltages used in this study. Applying the two-voltage technique for the A1SL chamber at its recommended operating voltage of 300 V leads to an overestimation of the recombination. Operating at a voltage of 100 V yields larger but more accurate values for the recombination correction. The recombination correction measured for this chamber in the TomoTherapy HiArt unit is lower than the 1% applied in the routine dosimetry for this treatment unit. For a helical dose delivery with a small slice width, lateral electron scatter in the cavity makes that the recombination is smaller than for an open beam delivering the same total dose. In a Farmer type chamber, a helical delivery with a 1 cm slice field results in a time and spatially integrated volume recombination of 55% of that with a 2.5 cm slice field. The relative recombination corrections for different slice widths and different field offsets with respect to the chamber center obtained from the developed models are in good agreement with experimental data.
Because of the presence of charge multiplication, it is more accurate to determine the recombination correction at lower operating voltages than are often applied using the two-voltage method. Models and experiments for partial irradiation conditions of the ion chamber show that resulting recombination corrections are reduced compared to those for an open field. A model for helical dose deliveries results in recombination corrections that get lower with smaller slice widths. This model could be adapted to any IMRT delivery where the ion chamber is instantaneously partial and/or inhomogeneously irradiated, and could provide a practical procedure to calculate the recombination for complex deliveries for which it is difficult to be measured.
在脉冲高能光子束中，电离室的离子复合是一个研究得很好的现象。尽管如此，由于使用高极化电压和简单的双电压方法的不当组合，离子复合的校正往往不准确。在新的治疗方式中，如调强放疗和螺旋断层放疗，会出现更多的情况，其中许多构成场的剂量学比单个静态场更相关。对于这些治疗方式，离子室几何形状的照射可能会瞬间不均匀和随时间变化。
本文研究了用于螺旋断层放疗束剂量测量的电离室中的离子复合。提出了用于连续束、脉冲大、小场和螺旋场中研究复合校正因子的模型。使用 Exradin A1SL、NE2571 和 NE2611 型电离室进行测量，并使用 PENELOPE 进行蒙特卡罗模拟，以支持这些模型。
对于本研究中使用的探测器和工作电压，在 60Co 和脉冲高能光子束中，发现初始复合和电荷倍增是相同的。在推荐的 300V 工作电压下对 A1SL 探测器应用双电压技术会导致复合的高估。在 100V 的工作电压下，得到的复合校正值较大，但更准确。在 TomoTherapy HiArt 装置中测量到的该探测器的复合校正值低于该治疗装置常规剂量测定中应用的 1%。对于小切片宽度的螺旋剂量输送，腔体内的侧向电子散射使得复合比输送相同总剂量的开放束小。在 Farmer 型探测器中，对于 1cm 切片场的螺旋输送，与 2.5cm 切片场相比，时间和空间积分体积复合降低了 55%。从开发的模型中获得的不同切片宽度和相对于探测器中心的场偏移的相对复合校正值与实验数据吻合较好。
由于存在电荷倍增，因此在使用双电压方法时，在较低的工作电压下确定复合校正更准确。用于离子室部分照射条件的模型和实验表明，与开放场相比，得到的复合校正值降低。螺旋剂量输送的模型导致复合校正值随着切片宽度的减小而降低。该模型可以应用于任何瞬间部分照射和/或不均匀照射的调强放疗输送，并且可以为难以测量的复杂输送提供计算复合的实用程序。
