基于实验和蒙特卡罗方法的两种电离室在高能光子场中磁场修正因子的确定。

Institute for Medical Physics and Radiation Protection, University of Applied Sciences Giessen, Giessen, Germany.

Department of Radiotherapy and Radiation Oncology, University Medical Center Giessen and Marburg, Marburg, Germany.

Med Phys. 2023 Jul;50(7):4578-4589. doi: 10.1002/mp.16345. Epub 2023 Mar 22.

BACKGROUND

The integration of magnetic resonance tomography into clinical linear accelerators provides high-contrast, real-time imaging during treatment and facilitates online-adaptive workflows in radiation therapy treatments. The associated magnetic field also bends the trajectories of charged particles via the Lorentz force, which may alter the dose distribution in a patient or a phantom and affects the dose response of dosimetry detectors.

PURPOSE

To perform an experimental and Monte Carlo-based determination of correction factors , which correct the response of ion chambers in the presence of external magnetic fields in high-energy photon fields.

METHODS

The response variation of two different types of ion chambers (Sun Nuclear SNC125c and SNC600c) in strong external magnetic fields was investigated experimentally and by Monte Carlo simulations. The experimental data were acquired at the German National Metrology Institute, PTB, using a clinical linear accelerator with a nominal photon energy of 6 MV and an external electromagnet capable of generating magnetic flux densities of up to 1.5 T in opposite directions. The Monte Carlo simulation geometries corresponded to the experimental setup and additionally to the reference conditions of IAEA TRS-398. For the latter, the Monte Carlo simulations were performed with two different photon spectra: the 6 MV spectrum of the linear accelerator used for the experimental data acquisition and a 7 MV spectrum of a commercial MRI-linear accelerator. In each simulation geometry, three different orientations of the external magnetic field, the beam direction and the chamber orientation were investigated.

RESULTS

Good agreement was achieved between Monte Carlo simulations and measurements with the SNC125c and SNC600c ionization chambers, with a mean deviation of 0.3% and 0.6%, respectively. The magnitude of the correction factor strongly depends on the chamber volume and on the orientation of the chamber axis relative to the external magnetic field and the beam directions. It is greater for the SNC600c chamber with a volume of 0.6 cm than for the SNC125c chamber with a volume of 0.1 cm . When the magnetic field direction and the chamber axis coincide, and they are perpendicular to the beam direction, the ion chambers exhibit a calculated overresponse of less than 0.7(6)% (SNC600c) and 0.3(4)% (SNC125c) at 1.5 T and less than 0.3(0)% (SNC600c) and 0.1(3)% (SNC125c) for 0.35 T for nominal beam energies of 6 MV and 7 MV. This chamber orientation should be preferred, as may increase significantly in other chamber orientations. Due to the special geometry of the guard ring, no dead-volume effects have been observed in any orientation studied. The results show an intra-type variation of 0.17% and 0.07% standard uncertainty (k=1) for the SNC125c and SNC600c, respectively.

CONCLUSION

Magnetic field correction factors for two different ion chambers and for typical clinical photon beam qualities were presented and compared with the few data existing in the literature. The correction factors may be applied in clinical reference dosimetry for existing MRI-linear accelerators.

背景

磁共振断层成像与临床线性加速器相结合，可在治疗过程中提供高对比度、实时成像，并促进放射治疗中的在线自适应工作流程。伴随的磁场通过洛伦兹力使带电粒子的轨迹发生弯曲，这可能会改变患者或体模中的剂量分布，并影响剂量计探测器的剂量响应。

目的

在高能光子场中，通过实验和基于蒙特卡罗的方法确定校正因子，以校正存在外部磁场时离子室的响应。

方法

使用德国国家计量研究院 PTB 的临床线性加速器和能够产生相反方向高达 1.5 T 的磁通密度的外部电磁铁，实验和蒙特卡罗模拟研究了两种不同类型的离子室（Sun Nuclear SNC125c 和 SNC600c）在强外部磁场中的响应变化。实验数据是在标称光子能量为 6 MV 的情况下获得的，实验模拟几何结构对应于实验设置，另外还对应于国际原子能机构 TRS-398 的参考条件。对于后者，使用两种不同的光子能谱进行了蒙特卡罗模拟：用于实验数据采集的线性加速器的 6 MV 能谱和商用 MRI-线性加速器的 7 MV 能谱。在每个模拟几何结构中，研究了外部磁场、射束方向和室方向的三个不同取向。

结果

SNC125c 和 SNC600c 电离室的蒙特卡罗模拟和测量结果吻合良好，平均偏差分别为 0.3%和 0.6%。校正因子的大小强烈依赖于腔室的体积以及腔室轴相对于外部磁场和射束方向的取向。SNC600c 腔室的体积为 0.6 cm，比 SNC125c 腔室的体积 0.1 cm 大，校正因子较大。当磁场方向和腔室轴相同时，且它们垂直于射束方向时，在 1.5 T 时，离子室的计算响应超过了 0.7(6)%（SNC600c）和 0.3(4)%（SNC125c），在 0.35 T 时，在标称射束能量为 6 MV 和 7 MV 时，计算响应超过了 0.3(0)%（SNC600c）和 0.1(3)%（SNC125c）。应优先选择这种腔室取向，因为在其他腔室取向中，可能会显著增加。由于保护环的特殊几何形状，在研究的任何取向中都没有观察到死区效应。结果表明，SNC125c 和 SNC600c 的标准不确定度（k=1）分别为 0.17%和 0.07%。