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Recanalization after coil embolization can be prevented by radiation emitted from 32P coils. We wanted to determine the upper limits of 32P activities that could be implanted onto coils with respect to the potential injury to nearby nerves, delay in organization of the clot, and effects on neointima formation and recanalization.We studied the effects of various 32P activities on recanalization and organization of thrombus after coil occlusion of canine arteries and on neointima formation at the neck of canine carotid bifurcation aneurysms. We also tested potential injury to nerves in the vicinity of radioactive or nonradioactive coils in 3 models: the brachial plexus (near proximal vertebral arteries) and the lingual nerve in a lingual artery bifurcation aneurysm model, both models being treated by radioactive or standard coil occlusion. Finally, we wrapped lingual nerves with nonradioactive or high-activity coils and studied their effects on lingual nerves and tongues. Results were assessed with a pathological scoring system and compared with Mann-Whitney and Kruskal-Wallis tests.No deleterious effect of radiation on nerves could be detected. Neointima formation was not hampered, scores of aneurysms treated with 32P-coils being significantly better when compared with treatments with standard coils (P=0.002). Arteries treated with high-activity coils (>3.39 μCi) showed absent recanalization but delayed organization of the clot at 3 months compared with low-activity or nonradioactive coils (P<0.05).β-Radiation can prevent recanalization after coil occlusion. We could not demonstrate any deleterious effects of radioactivity on nervous structure or on neointima formation. Delayed organization of thrombus provides a rational basis to establish an upper limit for 32P activities to be implanted onto coils.