Morphometric and histological changes in the vascular wall after external radiation for the prevention of intimal hyperplasia

BACKGROUND: Recent reports describe spontaneous dissections and aneurysms after coronary and peripheral artery irradiation for the prevention of intimal hyperplasia. We investigated histological changes and the vasomotor reaction in the vascular wall after external radiation for the prevention of intimal hyperplasia in rabbits. MATERIALS AND METHODS: The aorta was experimentally injured in 34 rabbits who were then assigned to one of three groups: irradiation with 20 Gy; with 25 Gy; and a control group with no irradiation. Before the arterial injury and 45 days later, vasomotor function was assessed with an intravascular ultrasound catheter. The aorta was resected for morphometric and histological studies. RESULTS: After injury and irradiation, vasomotor responses were significantly lower in the two irradiated groups (P < 0.05). Intimal thickness and the intima/media ratio were significantly lower in irradiated groups. In the irradiated group histological examination showed reduced intimal proliferation with an intact endothelium. In the 20-Gy irradiated group the vascular media contained necrotic areas, and in the 25-Gy irradiated group, severe fibrosis. CONCLUSION: After arterial injury, external irradiation at 20 and 25 Gy effectively reduces aortic intimal and medial thickening. Histological changes include recasting with necrosis and fibrosis causing a decreased vasomotor response. Further investigations are needed to confirm medial necrosis and replacement with fibrosis. Because the irradiation doses in this study match those currently used and also recommended for experimental and clinical use, if confirmed in humans parietal recasting might possibly explain the reported spontaneous dissections and aneurysm formation after irradiation.