The dose distribution around the edges of an improved /sup 32/P source for endovascular brachytherapy: a Monte Carlo study

Endovascular brachytherapy has proven to be an effective tool in the reduction of restenosis following balloon angioplasty for coronary heart disease, while the range of optimal doses and precise target tissue are still unknown. However, over-dosing has been associated with increased thrombogenicity and permanent damage to vessel walls. Under-dosing has been found to stimulate neointima formation, causing restenosis. It has been suggested that restenosis near the edges (edge effect) of the intervention may be caused by the dose falloff towards near the source ends. Monte Carlo code EGS4 was used to calculate the dose distribution around the ends of a Guidant /sup 32/P source. At the ends of the source the 100% isodose curve was found to bend back toward the source starting at about 3 mm from the end of the active source (prescription to 2 mm radius). The 80% isodose line crossed the prescription line 1 mm from the source ends. Further calculations predicted improvement can be achieved by changing the specific activity within 1 mm of the ends of the source. A four-fold increase in specific activity in 0.5 mm segments at the ends of the source, followed by a 0.5 mm gap between the ends and the middle part of the source, kept the 100% isodose line at the prescription distance to within 0.5 mm of the active length of the source without any over-dosing. With five times the activity in the terminal segments, a "full length source" is achieved, involving a small over-dosing.