Graduate School of Health Sciences, Kumamoto University, 4-24-1 Kuhonji, Kumamoto, Japan.
Department of Health Sciences, Faculty of Life Sciences, Kumamoto University, 4-24-1 Kuhonji, Kumamoto, Japan.
Phys Med. 2019 Mar;59:112-116. doi: 10.1016/j.ejmp.2019.03.011. Epub 2019 Mar 14.
The aim of this study is to investigate the perturbation effect of parallel-plate ionization chambers on the buildup dose measurement in transverse magnetic fields, using Monte Carlo (MC) simulation. The NACP-02 and ROOS parallel-plate chambers and a PTW31010 cylindrical chamber were modeled for buildup dose measurement in magnetic fields, using the EGSnrc/cavity code. The irradiation condition was set to a 10 × 10 cm field in a water phantom at a source-to-surface distance (SSD) of 100 cm, using 6-MV photon spectrum. Magnetic fields of 0 0.35, 1.0, 1.5, and 3.0 T were applied perpendicularly to the direction of the photon beam. The overall perturbation factor P for the ionization chambers in the magnetic fields was also calculated. The dose to water was enhanced with increasing the magnetic field strength at a depth of less than 1 cm. Over a depth of 1.5 cm, there was no significant difference in the depth doses with and without magnetic field in water. The maximum depth dose (%) for the NACP-02 and ROOS chambers at 1.5 T was higher up to 12% and 14% than the maximum depth dose at 0 T, respectively. The depth dose curves of a PTW31010 chamber have a similar tendency to those of water. The P values for each chamber were the largest at the phantom surface. The transverse magnetic field has a greater effect on the dose response of the NACP and ROOS chambers than that of the PTW31010 chamber in the buildup region.
本研究旨在通过蒙特卡罗（MC）模拟探讨平行板电离室在横向磁场中对体模中建成剂量测量的干扰效应。使用 EGSnrc/cavity 代码对 NACP-02 和 ROOS 平行板电离室和 PTW31010 圆柱形电离室进行建模，以在磁场中进行建成剂量测量。照射条件设置为在水模体中源皮距（SSD）为 100 cm 的 10×10 cm 射野，使用 6-MV 光子能谱。在光子束的方向垂直施加 0、0.35、1.0、1.5 和 3.0 T 的磁场。还计算了电离室在磁场中的整体干扰因子 P。随着磁场强度的增加，水的剂量在小于 1 cm 的深度处增加。在 1.5 cm 深度以上，水中有无磁场的深度剂量无显著差异。在 1.5 T 时，NACP-02 和 ROOS 室的最大深度剂量（%）比 0 T 时的最大深度剂量分别高 12%和 14%。PTW31010 室的深度剂量曲线与水的深度剂量曲线具有相似的趋势。各室的 P 值在体模表面最大。在建成区，横向磁场对 NACP 和 ROOS 室的剂量响应的影响大于对 PTW31010 室的影响。
