磁场对Bebig型60Co HDR近距离治疗源剂量分布影响研究

目的 计算MRI引导近距离治疗中磁场对Bebig型⁶⁰Co HDR放射源周围剂量以及碰撞比释动能分布的扰动,判断在不同强度磁场下应用此型号放射源进行近距离治疗可行性。方法 基于蒙特卡洛Geant4软件,建立Bebig型⁶⁰Co近距离治疗源模型,然后置其于尺寸为10cm×10cm×10cm的均匀水模体中,并沿着x、y、z轴向划分大小为0.2mm×0.5mm×0.5mm的体元。在x轴向上分别施加磁场强度为0T、1.5T、3.0T均匀磁场,分别计算x轴向上距离治疗源中心10.0mm范围内碰撞比释动能与剂量分布以及两者比值随距离放射源中心远近变化情况。结果 1.5T的磁场对⁶⁰Co HDR治疗源附近空间范围内剂量分布无影响,而3.0T的磁场导致距离放射源中心r<6mm范围内剂量明显偏高,r=5.4mm处剂量增加了接近40%。碰撞比释动能与剂量比值在1.2mm结论 对于Bebig型⁶⁰Co HDR近距离治疗源,在治疗过程中应用1.5T外部磁场是安全可靠的,而3.0T高场强磁场带来了剂量风险,在投入临床使用前必须进行安全验证评估。

The impact of a magnetic field on the dose distribution using the Bebig60Co HDR sources

Objective To evaluate the impact of an external magnetic field on the dose distribution and electronic disequilibrium region around a Bebig type ⁶⁰Co HDR brachytherapy source and to judge the feasibility of applying MRI scanner during brachytherapy.Methods The source model was established based on the Monte Carlo package Geant4 software. The simulated geometries consisted of the ⁶⁰Co source inside a water phantom of 10.0cm×10.0cm×10.0cm in size. The magnetic field strength of the 0T,1.5T and 3.0T was considered,respectively. The voxels with a size of 0.2 mm, 0.5 mm and 0.5 mm were established along the x-, y-and z-axis. The influence of the magnetic field on the Kerma (kinetic energy released to matter) distribution and dose distribution within the 10.0mm region from the source center was evaluated. Furthermore,the ratio of the Kerma to dose as a function of the distance to the center source was acquired. Results The 1.5T magnetic field exerted no effect on the dose distribution adjacent to ⁶⁰Co HDR brachytherapy source, whereas3.0T magnetic field caused significant increase in the dose distribution within r<6 mm from the source center. The dose distribution was increased by 40% at r=5.4 mm from the source center. The ratio of Kerma to dose was less than 1 within the region of 1.2 mmConclusions For Bebig ⁶⁰Co HDR brachytherapy source,it is safe and reliable to apply1.5T external magnetic field. Nevertheless, 3.0T magnetic field can cause high dose risk. Consequently, safety assessment and verification should be delivered prior to clinical application.