Chapter 41

### INTRODUCTION

With the growing popularity of peripheral vascular medicine, identifying a reliable treatment to the plaguing recurrence of restenosis will increase and augment the benefits of vascular intervention. Investigators have shown that the endovascular delivery of radiation therapy is one such treatment. Combating restenosis in the peripheral vascular system is contingent upon understanding the processes, mechanisms, and potential targets affected by using brachytherapy. The successful outcome of clinical trials in the coronary arteries facilitated recognition of vascular brachytherapy (VBT) to become standard of care for the treatment of in-stent restenosis (ISR). Expansion of the indications to de novo lesions identified the potential but also limitations of the technology (late thrombosis and edge effect). Simultaneously, investigators embarked on a series of studies utilizing VBT as adjunct therapy for intervention in peripheral arteries.

As patients in the baby boomer generation near their 60s, the full impact of peripheral and coronary atherosclerosis in the United States is apparent. Whereas coronary vascular procedures increase at a rate of 8% per year, there is greater growth in the frequency of peripheral procedures, estimated at 19% per year. Despite new advances such as drug-eluting stents, atherectomy devices, thrombectomy, and endoluminal grafts, the restenosis rate after peripheral artery intervention continues to compromise the overall success of these procedures.

Restenosis is still considered the "Achilles heel" of percutaneous endovascular intervention.1,2,3,4,5,6,7,8 Among the approaches for restenosis prevention and treatment in the peripheral arterial system (PAS), only VBT is reported to be safe and effective in this select group of patients. This article reviews the status of VBT, the available systems and dosimetry for use, and provides a summary of the latest reports from the clinical trials utilizing VBT to prevent or treat restenosis in the PAS.

### RADIATION SYSTEMS FOR THE PERIPHERAL VASCULAR SYSTEM

The vessel size of the PAS favored the use of gamma radiation because of the penetration characteristics of the emitter. The majority of investigational work performed in the PAS used Ir-192 in doses of 14 to 18 Gy prescribed at 2 mm from the source center.

Gamma rays are photons that originate from the center of the nucleus, as opposed to X-rays, which originate from the orbital outside of the nucleus. Gamma rays have deeply penetrating energies between 20 keV and 20 MeV, which require an excess of shielding, as compared to beta and X-ray emitters. The only gamma-ray isotope currently in use is Ir-192. Other isotopes which emit both gamma- and X-rays are Iodine-125 (I-125) and Palladium-103 (Pd-103), which have lower energies and require higher activities to deliver the prescribed dose in an acceptable dwell time (<20 min). The latter isotopes are either not available in such activities or too expensive for this application. The dosimetry of Ir-192 is well understood and as a result of the lesser fall-off in dose compared with beta emitters, the dose gradient at the area of interest is acceptable. Ir-192 is available in activities of up to 10 Ci, but because of the high penetration, the average shielding of a catheterization laboratory will not be able to handle more than 500-mCi source in activity. This limitation is associated with dwell times >12 minutes for doses >15 Gy when prescribed at 2-mm radial distance from the source.

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