Shielding effects of metallic encapsulations and radiographic contrast agents for catheter-based intravascular brachytherapy

Abstract

Purpose/Objective: Both photon- and beta-emitting radionuclides for intravascular brachytherapy (IVB) are under active investigation for prevention of restenosis following conventional angioplasty with or without stents. High atomic number materials are usually present in the coronary vessels undergoing treatment in the form of metallic encapsulations, stents, calcified plaque, or radiographic contrast agent. The high atomic number materials are likely to interfere with the photons and betas and, thus, change the dosimetry in the treatment volume. The purpose of this study is to investigate the shielding effects caused by the presence of high atomic number materials in IVB. Materials and Methods: Dose rates at various distances in water, with and without the presence of various high atomic number materials, were calculated using Monte Carlo simulation techniques for photon and electron transport in extended media. The high atomic number materials investigated included titanium, stainless steel, calcified plaque, Hypaque, and Omnipaque. A wide range of monoenergetic photon and electron sources and several photon- and beta-emitting radionuclides, which have been under consideration for IVB, were used. The energy of the monoenergetic photon sources was in the range from 10 keV to 1 MeV, and that of the monoenergetic electron sources in the range from 0.5 to 2 MeV. Photon-emitting radionuclides 192Ir, 125I, and 103Pd and beta-emitting radionuclides 90Y, 32P, and 188Re were also considered. Results: It was found that the high atomic number materials interfere considerably with the transport of photons of relatively low energies (below 40 keV) and all electron sources. When the energy of photon exceeds 100 keV, the interference becomes minimum for the high atomic number materials that are likely to be present in clinical situations. The shielding correction factors (SCFs) for dose rate at 2 mm from center were essentially 1.00 for photon energies above 100 keV; as the energy decreased below 100 keV, the SCF became smaller reaching a value of almost 0 for the lowest energy studied, 10 keV. For the photon source of 192Ir, the SCF was essentially 1.00; while for the photon sources of 125I and 103Pd, shielding corrections were considerably lower than 1.00 depending on the type and thickness of the high atomic number material. For the beta emitting sources, the shielding effect can be expressed as a loss in effective penetration depth. This loss depends both on the material and its thickness. For titanium and stainless steel, the loss of range was about two and four times the thickness of the metal. Conclusions: The effects of high atomic number materials, such as metallic stents, calcified plaque, and contrast agents are minimal for high energy photon emitters, such as 192Ir. The effects are pronounced for beta emitters and low energy photon emitters, and must be included in dosimetry planning.