临床电子束百分深度剂量测量中圆柱形电离室的应用可能性。
Possibility of using cylindrical ionization chambers for percent depth-dose measurements in clinical electron beams.
机构信息
Graduate School of Health Sciences, Kumamoto University, 4-24-1 Kuhonji, Kumamoto 862-0976, Japan.
出版信息
Med Phys. 2011 Aug;38(8):4647-54. doi: 10.1118/1.3608903.
PURPOSE
This study investigated the possibility of using cylindrical ionization chambers for percent depth-dose (PDD) measurements in high-energy clinical electron beams.
METHODS
The cavity correction factor, P(cav), for cylindrical chambers with various diameters was calculated as a function of depth from the surface to R50, in the energy range of 6-18 MeV electrons with the EGSnrc C(++) -based user-code CAVITY. The results were compared with those for IBA NACP-02 and PTW Roos parallel-plate ionization chambers. The effective point of measurement (EPOM) for the cylindrical chamber and the parallel-plate chamber was positioned according to the IAEA TRS-398 code of practice. The overall correction factor, P(Q), and the percent depth-ionization (PDI) curve for a PTW30013 Farmer-type chamber were also compared with those of NACP-02 and Roos chambers.
RESULTS
The P(cav) values at depths between the surface and R50 for cylindrical chambers were all lower than those with parallel-plate chambers. However, the variation in depth for cylindrical chambers equal to or less than 4 mm in diameter was equivalent to or smaller than that for parallel-plate chambers. The P(Q) values for the PTW30013 chamber mainly depended on P(cav), and for parallel-plate chambers depended on the wall correction factor, P(waII), rather than P(cav). P(Q) at depths from the surface to R50 for the PTW30013 chamber was consequently a lower value than that with parallel-plate chambers. However, the variation in depth was equivalent to that of parallel-plate chambers at electron energies equal to or greater than 9 MeV. The shift to match calculated PDI curves for the PTW30013 chamber and water (perturbation free) varied from 0.65 to 0 mm between 6 and 18 MeV beams. Similarly, the shifts for NACP-02 and Roos chambers were 0.5-0.6 mm and 0.2-0.3 mm, respectively, and were nearly independent of electron energy.
CONCLUSIONS
Calculated PDI curves for PTW30013, NACP-02, and Roos chambers agreed well with that of water by using the optimal EPOM. Therefore, the possibility of using cylindrical ionization chambers can be expected for PDD measurements in clinical electron beams.
目的
本研究旨在探讨圆柱形电离室在高能临床电子束百分深度剂量（PDD）测量中的应用可能性。
方法
利用 EGSnrc C++ 为基础的用户代码 CAVITY，计算了不同直径圆柱形电离室的空腔校正因子 P(cav)，作为从表面到 R50 深度的函数，能量范围为 6-18 MeV 电子。将结果与 IBA NACP-02 和 PTW Roos 平行板电离室进行比较。根据 IAEA TRS-398 实践规范，将圆柱形电离室和平行板电离室的有效测量点（EPOM）定位。还比较了 PTW30013 Farmer 型电离室的整体校正因子 P(Q)和百分深度电离（PDI）曲线与 NACP-02 和 Roos 电离室的曲线。
结果
在表面和 R50 之间的深度处，圆柱形电离室的 P(cav)值均低于平行板电离室的 P(cav)值。然而，直径等于或小于 4mm 的圆柱形电离室的深度变化与平行板电离室的变化相当或更小。PTW30013 室的 P(Q)值主要取决于 P(cav)，而平行板电离室的 P(Q)值则取决于壁校正因子 P(waII)，而不是 P(cav)。因此，在表面到 R50 深度处，PTW30013 室的 P(Q)值低于平行板电离室的值。然而，在电子能量等于或大于 9 MeV 时，深度变化与平行板电离室的变化相当。为了使 PTW30013 室和水（无扰）的计算 PDI 曲线相匹配，在 6-18 MeV 束之间的变化范围为 0.65-0mm。类似地，NACP-02 和 Roos 室的变化分别为 0.5-0.6mm 和 0.2-0.3mm，并且几乎与电子能量无关。
结论
通过使用最佳 EPOM，PTW30013、NACP-02 和 Roos 室的计算 PDI 曲线与水的 PDI 曲线吻合良好。因此，圆柱形电离室在临床电子束 PDD 测量中具有应用的可能性。
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