Department of Radiological Sciences, Komazawa University Graduate School, Setagaya-ku, Tokyo, Japan.
Department of Radiation Oncology, Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital, Bunkyo-ku, Tokyo, Japan.
J Appl Clin Med Phys. 2023 Jul;24(7):e14059. doi: 10.1002/acm2.14059. Epub 2023 Jun 12.
In this study, the effective point of measurement (EPOM) for plane-parallel ionization chambers in clinical high-energy electron beams was determined experimentally. Previous studies have reported that the EPOM of plane-parallel chambers is shifted several tens of millimeters downstream from the inner surface of the entrance window to the cavity. These findings were based on the Monte Carlo (MC) simulation, and few experimental studies have been performed. Thus, additional experimental validations of the reported EPOMs were required. In this study, we investigated the EPOMs of three plane-parallel chambers (NACP-02, Roos and Advanced Markus) for clinical electron beams. The EPOMs were determined by comparing the measured percentage depth-dose (PDD) of the plane-parallel chambers and the PDD obtained using the microDiamond detector. The optimal shift to the EPOM was energy-dependent. The determined EPOM showed no chamber-to-chamber variation, thereby allowing the use of a single value. The mean optimal shifts were 0.104 ± 0.011, 0.040 ± 0.012, and 0.012 ± 0.009 cm for NACP-02, Roos, and Advanced Markus, respectively. These values are valid in the R range from 2.40 to 8.82 cm, which correspond to 6-22 MeV. Roos and Advanced Markus exhibited similar results to those of the previous studies, but NACP-02 showed a larger shift. This is probably due to the uncertainty of the entrance window of NACP-02. Therefore, it is necessary to carefully consider where the optimal EPOM is located when using this chamber.
在这项研究中，通过实验确定了临床高能电子束中平行平板电离室的有效测量点（EPOM）。先前的研究报告称，平行平板室的 EPOM 从入口窗的内表面到腔体内下游移动了几十毫米。这些发现是基于蒙特卡罗（MC）模拟得出的，很少有实验研究。因此，需要对报道的 EPOM 进行额外的实验验证。在这项研究中，我们研究了三种用于临床电子束的平行平板电离室（NACP-02、Roos 和 Advanced Markus）的 EPOM。通过比较平行平板电离室的测量百分深度剂量（PDD）和使用 microDiamond 探测器获得的 PDD，确定了 EPOM。EPOM 的最佳偏移量是能量依赖性的。确定的 EPOM 没有显示出室间变化，因此可以使用单个值。NACP-02、Roos 和 Advanced Markus 的平均最佳偏移量分别为 0.104±0.011、0.040±0.012 和 0.012±0.009 cm。这些值在 R 范围为 2.40 至 8.82 cm 内有效，对应于 6-22 MeV。Roos 和 Advanced Markus 的结果与先前的研究相似，但 NACP-02 的偏移量更大。这可能是由于 NACP-02 入口窗的不确定性造成的。因此，在使用该室时，有必要仔细考虑最佳 EPOM 的位置。
