Intravascular brachytherapy physics: introduction

Coronary artery disease is the most common cause of death in the U.S. Treatment modalities include bypass surgery and Percutaneous Transluminal Coronary Angioplasty (PTCA). The latter is less morbid, less expensive, and has an initial success rate of 90%. However, there is a 30-50% restenosis rate within 6 months. The biological mechanisms of restenosis are not fully understood, but it is believed to be caused by neointimal growth and vascular remodeling. The latter can be reduced by implantation of stents, but stents do not reduce neointimal growth. Intravascular Brachytherapy (IVB) is the use of temporary or permanent radioactive implants to prevent neointimal growth and restenosis following PTCA. Prescribed doses are typically 8-30 Gy to a point 1-2 mm from the source, or 0.5-1.5 mm depth into the vessel wall. Because of the steep dose gradients doses to other points may differ significantly from the prescribed dose. The goal of treatment is to deliver a large enough dose to all target cells so as to inhibit new cell growth without exceeding normal tissue tolerance which could lead to vascular necrosis. To date, catheter based IVB, wherein radioactive seeds or wires are temporarily inserted through coronary catheters has been the most common method of IVB. Dose rates are typically >1 Gy/min and large activities of radioisotopes are required (>300 mCi for photon emitters, >20 mCi for betas). The permanent implantation of radioactive stents is an alternative method of IVB