The arteries carry blood from the heart to various parts of the body. The main artery, the aorta, is the largest blood vessel (2–3 cm in diameter) and carries blood from the left ventricle of the heart to the branching arteries and capillaries in all the body organs and tissues, including the heart muscle. The coronary arteries are blood vessels lying on the outer surface of the heart and the peripheral arteries supply blood to other organs and tissues. The luminal diameter of arteries ranges from 3 to 5 mm initially and tapers slowly through their path length. Within this range, the peripheral arteries tend to be of larger diameter than the coronary arteries. The minimum normal artery diameter required for angioplasty and stenting is approximately 3 mm.

The inside of the arteries is lined with a layer of cells called endothelium. Next to the endothelium is the connective tissue layer, the intima, followed by layers of elastic membrane, smooth muscle cells, and elastic tissues. The outermost layer of the arteries is called adventitia, made up chiefly of collagenous fiber.

The arteries can be partially blocked due to atherosclerosis or plaque formation. Reduction of their lumen diameter compromises the flow of blood and the delivery of oxygen to the body tissues. As an alternative to a major surgical procedure such as bypass surgery, balloon angioplasty is used to dilate the lumen diameter. This stretching action often ruptures the internal elastic lamina of the wall and causes fissures in the medial layers. The acute risk of the angioplasty procedure is thrombosis that can be controlled by drugs, as mentioned previously. The more protracted risk, however, is that of restenosis by neointimal hyperplasia. This process involves growth of new tissues in the
cracks and crevices of the arterial wall caused by angioplastic injury. Although implantation of stent angioplasty reduces the overall rate of restenosis by approximately 50%, it does not prevent neointimal growth and may, in fact, stimulate the process.

Target volume for intravascular brachytherapy (IVBT) is confined to the region of angioplasty. Typically, it is 2 to 5 cm in length of artery and 0.5 to 2 mm in thickness of arterial wall. Occasionally, these dimensions may be exceeded depending on the location and extent of the disease. With 3 to 5 mm of luminal diameter, the radial range of treatment may extend as far as to about 5 mm from the center of the artery.

Because of the severity of inverse square falloff of radiation at short distances, transluminal irradiation with intravascular brachytherapy produces highly conformal dose distribution, delivering a high dose to the arterial wall while sparing surrounding normal vessels or myocardium. Again, because of the predominance of the inverse square law effect, penetrating power of radiation, depending on energy and modality, is not critically important, except with regard to dose rate or duration of implant and radiation protection of personnel involved with the procedure. β-Particle sources, in general, give higher dose rates and provide greater radiation protection compared to the γ-ray sources.

The depth of dose prescription for intracoronary irradiation is recommended by the American Association of Physicists in Medicine (AAPM) (6) to be 2 mm from the center of the source and for the peripheral arteries 2 mm beyond the average lumen radius. For each case, dose distribution in at least three planes perpendicular to the catheter and along its length should be determined. In addition, average, maximum, and minimum doses within the target volume should be reported (6).

Typical dosimetric requirements of a temporary intravascular implant are (a) to deliver a target dose of 15 to 20 Gy to a 2- to 3-cm length of the arterial wall involved at a radial distance of about 2 mm from the source center, (b) to minimize the dose to tissues outside the region of angioplasty, and (c) to take as little time as possible for completion of the procedure, that is, provide target dose rates on the order of 5 Gy/min or greater. These requirements suggest the suitability of high-energy β sources such as strontium-90, yttrium-90, and phosphorus-32 or high-activity γ sources such as iridium-192. The latter could be a high-dose-rate (HDR) afterloading unit with the source dimensions small enough to allow intravascular brachytherapy.

The β sources have several advantages over γ sources: higher specific activity, higher dose rate, longer half-life, and greater radiation safety for the patient as well as personnel. The major disadvantage of β sources, however, is the extremely rapid radial dose falloff within the target region; γ sources such as ¹⁹²Ir provide relatively more uniform target dose, governed primarily by the inverse square law falloff with distance, but require high activity to yield a reasonably high dose rate (≥5 Gy/min). Consequently, radiation protection problems with such sources become more significant. Although the HDR afterloaders using γ sources could provide sufficiently high dose rate, they would require expensive shielding of the catheterization laboratories.

Table 24.1 contains a list of possible isotopes that have been or could be used for intravascular brachytherapy. The last column shows the activities required to obtain a dose rate of 5 Gy/min to a 2-cm length of a vessel at a radial distance of 2 mm from the source center. It is seen that the γ sources, because of lower specific activity, require much higher activities than the β sources for a catheter-based intravascular procedure. On the other hand, a permanent radioactive stent using ⁴⁸V requires only 1 μCi to produce the same dose rate.

Although dose rate per unit-activity favors β emitters, radial dose distribution is better for the γ sources, if it is assumed that the dose uniformity across the target volume is radiobiologically beneficial. This assumption has not been clinically validated but it seems logical, based on experience in
conventional radiation therapy. Figure 24.1 compares radial dose distribution as a function of radial distance for some of the sources listed in Table 24.1. Nath and Liu (7) have studied radial dose function (g) for point sources of photons and electrons using Monte Carlo simulation. Their data show that from the point of view of adequate depth of penetration for intravascular brachytherapy, photon sources above 20 keV and electron sources above 1.0 MeV are acceptable.

Irradiation of blood vessels to prevent restenosis following angioplasty has been carried out using external beam as well as brachytherapy. Current trends favor catheter-based endovascular brachytherapy devices. While research continues to develop new sources and delivery techniques, a number of systems have become available commercially. Of these, the U.S. Food and Drug Administration has approved only a few for clinical use. A brief review of some of the available devices is presented below. For more detailed product information and specifications, the reader is referred to the respective company literature.