Abstract: Objective :To evaluate the role of external beam radiation combined with interstitial brachytherapy for nasopharyngeal carcinoma. Methods :Ninety-eight cases with nasopharyngeal carcinoma had residual lesion left in the parapharygeal space,as confirmed by CT or MRI after a standard dose of 68-70Gy with conventional external irradiation.They were divided into two groups randomly.One with 47 cases(PSIBT group)received interstitial brachytherapy to give a boost complement.With 1-3 implanted catheters,an additional boost of 12-15Gy/6-8 fractions/2-3 days was completed by after-loading of high dose rate ¹⁹²Ir wires.The other group(51 cases)received an additional dose of 12-14Gy with external beam radiotherapy as control. Results :There were no statistical significance(P>0.05)in the overall 3-year survival rates and the 3-year distant metastasis-free survivals for two groups.The 3-year local relapse-free survival of PSIBT group was higher than that of control group(P<0.05).And in the PSIBT group,the chronic sequelae,such as xerostomia,decreased hearing,trismus etc,happened less than that of the control group significantly(P<0.05). Conclusion :As the boost for nasopharyngeal cancer at parapharyngeal space,the inserted after-loading therapy can obviously enhanced the local control rate for residues in the parapharyngeal space,and the effect of irradiation on tumors of advanced stage is significantly lower compared to boost complement at post aurem field,with a relatively ideal value of clinical application.