Graduate school of Health Sciences, Kumamoto University, 4-24-1 Kuhonji, Chuo-ku, Kumamoto, 862-0976, Japan.
Department of Health Sciences, Faculty of Life Sciences, Kumamoto University, 4-24-1 Kuhonji, Chuo-ku, Kumamoto, 862-0976, Japan.
Med Phys. 2020 Apr;47(4):1995-2004. doi: 10.1002/mp.14054. Epub 2020 Feb 20.
The purpose of this study was to investigate the impact of transverse magnetic fields on the dose response of a radiophotoluminescent glass dosimeter (RGD) in megavoltage photon beams.
The RGD relative response (i.e., RGD dose per absorbed dose to water at the midpoint of the detector in the absence of the detector) was calculated using Monte Carlo (MC) simulations. Note that the Monte Carlo calculations do not account for changes of the signal production per unit dose to the RGD caused by the magnetic field strength. The relative energy response R , the relative magnetic response R , and the relative overall response R with the transverse magnetic fields of 0-3 T were analyzed as a function of depth, for a 10 cm × 10 cm field in a solid water phantom, for 4-18 MV photons. Although magnetic resonance (MR) linacs with flattening filter free beams are commercially available, flattening filter beams were used to investigate the RGD response in this study. R is the response in beam quality Q relative to that in the reference beam with quality 6 MV, R is the response in beam quality Q with the magnetic field relative to that in beam quality Q without the magnetic field, and the R is the response in beam quality Q with the magnetic field relative to that in the reference beam with quality 6 MV without the magnetic field. Two RGD orientations were considered: RGD long axis is parallel (direction A) and perpendicular (direction B) to the magnetic field. The reference irradiation conditions were at the depth of 10 cm for a 10 cm × 10 cm field for 6 MV, without the magnetic field. In addition, the influence of a small air-gap between the holder inner wall and the RGD on the dose response in the magnetic field, R , was analyzed in detail. R is the response in beam quality Q without/with the air-gap.
R decreased by up to 2.7% as the energy increased in the range of 4-18 MV, except in the buildup region. In direction A, the variation of R owing to the magnetic field strength was below 1.0%, regardless of the photon energy. In contrast, in direction B, R decreased with increasing magnetic field strength and decreased up to 4.0% at 3 T for 10 MV. The R for 0.03 and 0.05 cm air-gap models in direction A decreased up to 2.3% and up to 4.0%, respectively.
The variation of R changed with the direction of the RGD relative to the magnetic field. For dose measurements, RGDs should be positioned with the long axis parallel to the magnetic field, without air-gaps.
本研究旨在探讨横向磁场对放射性光致发光玻璃剂量计（RGD）在兆伏光子射束中剂量响应的影响。
使用蒙特卡罗（MC）模拟计算 RGD 的相对响应（即探测器中点处探测器无水吸收剂量的 RGD 剂量）。需要注意的是，蒙特卡罗计算并未考虑磁场强度引起的 RGD 每单位剂量信号产生的变化。分析了磁场强度为 0-3T 时的相对能量响应 R、相对磁场响应 R 和相对总响应 R 与深度的关系，在固体水模体中，对于 10cm×10cm 的场，用于 4-18MV 光子。尽管具有无均整滤波器的磁共振（MR）直线加速器已商业化，但在本研究中使用了均整滤波器束来研究 RGD 响应。R 是在束质 Q 中的响应相对于在参考束质 6MV 中的响应，R 是在磁场中的束质 Q 中的响应相对于在无磁场中的束质 Q 中的响应，R 是在磁场中的束质 Q 中的响应相对于在无磁场中的参考束质 6MV 中的响应。考虑了两种 RGD 取向：RGD 长轴平行（方向 A）和垂直（方向 B）于磁场。参考照射条件为在 10cm 深度处 10cm×10cm 的场，6MV，无磁场。此外，详细分析了磁场中剂量响应 R 与 holder 内壁和 RGD 之间的小气隙的影响。R 是在无/有气隙的束质 Q 中的响应。
在 4-18MV 的范围内，R 随着能量的增加而降低，最大降低幅度为 2.7%，除了在堆积区。在方向 A 中，磁场强度的变化小于 1.0%，与光子能量无关。相比之下，在方向 B 中，R 随着磁场强度的增加而减小，在 3T 时对于 10MV 减小了 4.0%。在方向 A 中，0.03 和 0.05cm 气隙模型的 R 分别降低了 2.3%和 4.0%。
R 的变化随 RGD 相对于磁场的方向而变化。对于剂量测量，RGD 应平行于磁场方向放置，无气隙。
