ABSTRACT
<p><b>OBJECTIVE</b>To study the effect of irradiation on the susceptibility of radiation caries.</p><p><b>METHODS</b>The structures of 56 teeth enamel and dentin of 63 roots were observed using SEM and the collagen fibre and the resistance to the acid were also investigated after irradiation of 30 Gy, 50 Gy and 70 Gy.</p><p><b>RESULTS</b>The enamel structure changes were found after irradiation with different doses. The significant difference was found in the enamel changes between high or middle dose group and low dose group or control. The dentin morphology changed, some collagen fibre vanished and resistance to acid was reduced after irradiation with 50 Gy and 70 Gy.</p><p><b>CONCLUSIONS</b>The radiation reduced the resistance of teeth to the acid and increased the caries susceptibility.</p>