Original articleShielding effects of metallic encapsulations and radiographic contrast agents for catheter-based intravascular brachytherapy
Introduction
Radiation, either delivered by photon emitters or beta emitters, has shown great potential for the treatment of restenosis following angioplasty [1], [2], [3], [4], [5], [6]. Typically, a dose of 15 Gy of ionizing radiation is prescribed at a distance of 2 mm from the center of a coronary blood vessel. The treatment distances in intravascular brachytherapy (IVB) are considerably smaller than those in traditional brachytherapy treatment. The distance between a target and radioactive source and the size of the target in IVB are normally in the range of millimeters. In a recent study, it was reported that photons of energy from 20 to 400 keV provide similar depth dose in arterial wall up to a distance of 10 mm [7]. On the other hand, for the electrons, the energy has to be at least higher than 1 MeV to provide sufficient depth dose in arterial wall. Clinically the existence of high atomic number materials complicates the issue of dosimetry. The high atomic number materials could be contrast agents that are usually injected into blood vessels to help in the determination of lesion location. The contrast agents, like Omnipaque and Hypaque, contain iodine, which has an atomic number (Z) of 53. Some patients have stents that are usually made of stainless steel or titanium. In many cases, plaques themselves are calcified. Furthermore, most radionuclides are encapsulated by metallic sheaths. The presence of high atomic number materials in the blood vessels could affect the dose and penetration of photon and beta particles. The objective of the current study was to investigate the shielding effects of high atomic number materials on the dosimetry at prescription point, typically at 2 mm from center. The effects at the interface itself (within distances of less than 0.1 mm) have been reported earlier [8], [9] and are not the topic of interest in this work.
Section snippets
Materials and methods
The detailed description of the dosimetry calculation has been described in a previous publication [7]. Briefly, Integrated TIGER Series (ITS) of Coupled Electron/Photon Monte Carlo Code System (Version 2.1) was used for dosimetry calculation. The ITS system was run on a DEC AlphaStation 200/66. The operating system was VMS Version 6.2. The cutoff energy was 1 keV for both photons and electrons. The number of histories was 1,000,000 for photons and 100,000 for electrons. For photon calculation,
Photon sources
Fig. 1 illustrates the effects of titanium encapsulation on the depth dose of selected monoenergetic photons and radionuclides 192Ir, 125I, and 103Pd. The thickness of titanium was 0.05, 0.1, and 0.2 mm, respectively. For monoenergetic photons, as shown in Fig. 1, the attenuation caused by titanium encapsulation of thickness up to 0.2 mm was negligible for photons of energy 100 keV and higher. However, even a 0.05-mm titanium encapsulation reduced the dose rate at 2-mm distance by about 4% for
Discussions and conclusions
The encapsulation of the source by a metallic sheath reduces the dose rate, especially for low energy photons and beta emitters. However, this is not necessarily a detrimental effect, provided that the shielding effects are incorporated into the calibration of dose rate from the source. Therefore, it is essential that the actual dose rate from the encapsulated source should be determined (measured or calculated), in order to calculate the treatment time necessary for a given delivered dose.
One
Acknowledgements
This paper is supported in part by Grant No. 5R01-HL58022 from the National Heart, Lung, and Blood Institute.
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