Nath R, Yue N, Weinberger J
Department of Therapeutic Radiology, Yale University School of Medicine, New Haven, Connecticut 06510, USA.
Cardiovasc Radiat Med. 1999 Apr-Jun;1(2):144-53. doi: 10.1016/s1522-1865(99)00007-4.
In intravascular brachytherapy, use of high atomic number materials, such as contrast agents and metallic stents, can introduce significant dose perturbations, especially for low energy photons. The purpose of this study is to investigate dose perturbation at the interfaces of high atomic number materials and tissue.
To investigate this issue, the radial dose functions across the interface between different materials and soft tissue were calculated by using Monte Carlo simulations. Various interfaces, including contrast agent to water, stainless steel to water, and bone (simulating a calcified plaque) to water, were investigated for photon energies between 20 keV and 1 MeV.
It was found that the dose to water near the interface is enhanced considerably by photons of energies between 0.020 and 0.200 MeV. For example, the maximum dose enhancement factors for the Hypaque-tissue interface ranged from 2.2 to 18.3 for photons in this energy range. The enhancement factor is almost equal to 1 for photon energy between 0.400 and 1.000 MeV. It appears that the maximum enhancement occurs around 60 keV. For 60-keV photons, the maximum dose enhancement factors are about 18.3, 18.7, 19.1, and 3.1 for Hypaque, Omnipaque, stainless steel, and calcified plaque, respectively. The dose enhancement decreases exponentially with distance from the interface. The affected tissue thickness is dependent on the photon energy. As expected, the higher the photon energy is, the larger is the affected tissue thickness. Depending on the type of interface and the energy of photons, the dose enhancement distance (defined as the thickness receiving more than twice the dose without interface) ranges from 1.3 to 72 microm for photons of energy from 0.020 to 0.100 MeV, respectively.
The existense of high atomic number materials could introduce significant dose enhancement at the interfaces between these materials and tissue. This dose enhancement can be higher than an order of magnitude for photon energies around 60 keV, and should be considered in evaluation of the efficacy of intravascular brachytherapy.
在血管内近距离放射治疗中，使用高原子序数材料，如造影剂和金属支架，会引入显著的剂量扰动，尤其是对于低能光子。本研究的目的是调查高原子序数材料与组织界面处的剂量扰动情况。
为研究此问题，通过蒙特卡罗模拟计算了不同材料与软组织界面处的径向剂量函数。研究了各种界面，包括造影剂与水、不锈钢与水以及骨（模拟钙化斑块）与水的界面，光子能量范围为20 keV至1 MeV。
发现在界面附近，能量在0.020至0.200 MeV之间的光子会使水的剂量显著增加。例如，在此能量范围内，Hypaque与组织界面的最大剂量增强因子在2.2至18.3之间。对于能量在0.400至1.000 MeV之间的光子，增强因子几乎等于1。最大增强似乎发生在60 keV左右。对于60 keV的光子，Hypaque、欧乃派克、不锈钢和钙化斑块的最大剂量增强因子分别约为18.3、18.7、19.1和3.1。剂量增强随离界面距离呈指数下降。受影响组织厚度取决于光子能量。正如预期的那样，光子能量越高，受影响组织厚度越大。根据界面类型和光子能量，剂量增强距离（定义为接受的剂量超过无界面时两倍的厚度）对于能量为0.020至0.100 MeV的光子分别为1.3至72微米。
高原子序数材料的存在会在这些材料与组织的界面处引入显著的剂量增强。对于60 keV左右的光子能量，这种剂量增强可能高于一个数量级，在评估血管内近距离放射治疗的疗效时应予以考虑。
