Dose perturbations by high atomic number materials in intravascular brachytherapy

Abstract

Purpose. 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. Methods. 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. Results. 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 μm for photons of energy from 0.020 to 0.100 MeV, respectively. Conclusions. 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.