Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality
Monte Carlo study of ionization chamber magnetic field correction factors as a function of angle and beam quality
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DOI：
10.1002/mp.12716
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发表时间：
2018-02-01
期刊：
影响因子：
3.8
通讯作者：
Rogers, D. W. O.
中科院分区：
文献类型：
--
作者：
Malkov, Victor N.;Rogers, D. W. O.
目的：利用EGSnrc蒙特卡罗模拟进行磁场剂量测定，确定最佳测量方向，计算32个圆柱形离子室和3个平行板离子室的光束质量转换因子，评估这些因子对光束质量和角度的依赖关系，并研究磁场对% dd(10)(x)和TPR1020的影响。方法：在磁场为0.35 T的Co-60束流和磁场为1.5 T的7 MV束流条件下，分别计算光束质量转换因子k(Q)（mag）和磁场转换因子k(B) = k(Q)(mag) /k(Q)作为腔室旋转的函数，两个磁场均垂直于光子束。通过使用整个几何体积或排除与距阀杆1毫米处相关的空气体积来改变腔室的敏感空气体积。k(B)和k(Q)（mag）因子使用四种临床光子光谱进行评估。用doxyznrc研究了6种光子光谱中% dd(10)(x)和TPR1020随磁场的变化规律。结果：当磁场垂直于光子束时，平行于磁场取向的腔室减少了磁场对腔室响应的影响（即每水剂量对空气的剂量），并且基本消除了未知敏感体积引起的变化。尽管较大的k(B)值与高z电极的腔室有关，但对于大多数圆柱形腔室，计算出的k(B)因子在1%以内。PP腔室的k(B)校正高达8.9%，与圆柱形腔室相比，具有更大的角灵敏度。圆柱形离子室的k(B)值与光束质量无关，高z电极室除外。当% dd(10)(x)值介于63.3%和73.8%之间时，当磁场垂直于光子束平行于腔室时，k(B)变化最多为（0.26 +/- 0.15）%。对于Co-60、7和25 MV的光子束，无磁场与垂直于光子束的1.5 T场之间的% dd(10)(x)差异分别为（0.04 +/- 0.10）%、（1.89 +/- 0.10）%和（6.20 +/- 0.10）%，而TPR1020的变化小于（0.36 +/- 0.10）%。采用ICRU-90建议的停止功率而不是ICRU-37，发现k(Q)（因此k(B)）的变化小于0.1%。结论：在场垂直于光子束的情况下，将腔室平行于磁场定向，可以减小磁场对腔室响应的影响，消除敏感体积未知的问题。k(B)和k(Q)（mag）的值可以使离子室剂量法在磁场中符合TG-51方案。PP腔室对磁场敏感，由于小角度变化引起的腔室响应变化使其不太可能用于临床参考剂量测定。TPR1020的稳定性作为磁场和光束质量的函数，使其成为磁场中最佳的光束质量指示器。(C) 2017年美国医学物理学家协会
Purpose: To use EGSnrc Monte Carlo simulations for magnetic field dosimetry to determine optimal measurement orientations, calculate beam quality conversion factors for 32 cylindrical and three parallel-plate (PP) ion chambers, evaluate the beam quality and angular dependence of these factors, and examine the magnetic field effects on % dd(10)(x) and TPR1020.Methods: Beam quality conversion factors, k(Q)(mag), and magnetic field conversion factors, k(B) = k(Q)(mag) /k(Q), are calculated as a function of chamber rotation for six cylindrical ionization chamber in either a Co-60 beam with a 0.35 T magnetic field or a 7 MV beam with a 1.5 T field, both magnetic fields are perpendicular to the photon beam. The chambers' sensitive air volumes are varied by either using the entire geometric volume or excluding the air volume associated with the first 1 mm away from the stem. The k(B) and k(Q)(mag) factors are evaluated using four clinical photon spectra. The variation in % dd(10)(x) and TPR1020 as a function of magnetic field for six photon spectra are studied using DOSXYZnrc.Results: When the magnetic field is perpendicular to the photon beam, orienting the chamber parallel with the magnetic field reduces the magnetic field effect on chamber response (i.e., dose to air per water dose) and variations due to the unknown sensitive volume are essentially eliminated. Calculated k(B) factors are within 1% of unity for the majority of cylindrical chambers, although larger k(B) values are associated with chambers with high-Z electrodes. PP chambers have k(B) corrections as large as 8.9% and have a larger angular sensitivity compared to cylindrical chambers. Values of k(B) for cylindrical ion chambers are independent of beam quality, except for chambers with high-Z electrodes. For % dd(10)(x) values between 63.3% and 73.8%, k(B) varies by at most (0.26 +/- 0.15)% when the magnetic field is perpendicular to the photon beam and parallel to the chamber. Differences in % dd(10)(x), between no magnetic field and with a 1.5 T field perpendicular to the photon beam are (0.04 +/- 0.10)%, (1.89 +/- 0.10)%, and (6.20 +/- 0.10)% for a Co-60, 7, and 25 MV photon beam, respectively, while TPR1020 shows less than (0.36 +/- 0.10)% change. Applying the ICRU-90 recommendations for stopping powers instead of ICRU-37 is found to change k(Q) (and hence k(B)) by less than 0.1%.Conclusions: Orienting the chamber parallel to the magnetic field when the field is perpendicular to the photon beam will minimize the effect of the magnetic field on chamber response, and eliminate the problem of the unknown sensitive volume. Values of k(B) and k(Q)(mag) can bring ion chamber dosimetry in magnetic fields in-line with the TG-51 protocol. PP chamber are sensitive to the magnetic field and variation in chamber response due to small angular changes makes them unlikely candidates for clinical reference dosimetry in magnetic fields. The stability in TPR1020, as a function of magnetic fields and beam qualities, makes it the best beam quality specifier in magnetic fields. (C) 2017 American Association of Physicists in Medicine