Vascular brachytherapy: what have we learned from intravascular ultrasound?
With the advent of intravascular brachytherapy (IVBT), a striking reduction in the rate of restenosis has been observed. The use of intravascular ultrasound (IVUS) during IVBT trials has shown many aspects and relevant pathophysiologic mechanisms following this practical therapy. Specifically, IVUS quantitative assessments have demonstrated a drastic inhibition of both neointimal formation and negative remodeling to be the predominant vascular response to IVBT. Moreover, IVBT has shown promise for challenging high-risk restenosis cases, such as patients with diabetes mellitus and smaller caliber target vessels. However, unexpected radiation-induced complications have also been discovered, as more patients have been treated for a broad class of lesion subsets. Edge effect, induced by catheter-based radiation, was seen to be due to geographic miss of the radiation source, advocating a new concept known as "radiation edge". Furthermore, late thrombosis, which is known to be strongly associated with new stent implantation following IVBT, may be avoided with novel antiplatelet agents. Two additional complications, whose clinical significance remains unclear, are unhealed dissection and late stent malapposition featured by IVUS qualitative assessment. Unhealed dissection was observed in half of radiated dissections and late stent malapposition has been seen for all radiation sources in a small percentage of cases at 6-12 months follow-up. Radiation sources, dosimetry, and delivery methods continue to improve and should ultimately translate to more effective treatment for the patient with atherosclerotic coronary disease.