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

https://doi.org/10.1016/S1522-1865(00)00085-8Get rights and content

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.

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.

References (9)

There are more references available in the full text version of this article.

Cited by (0)

View full text