Abstract: Objective To study the impact of setup errors on treating head and neck cancer (HNC) with intensity modulated radiation therapy (IMRT).Methods Patients were scanned with KV-CBCT to adjust position before treatment. The setup errors including translational errors and rotation errors were gain from XVI (X-Ray volume Image) matched with original planed CT image. The setup errors were input to pinnacle treatment planning system，and then the treatment plans with setup errors were recalculated. The dose distribution and dose volume histogram (DVH) of tumor and critical structure were compared with original treatment plan.The evaluation indicators included mean dose (Dm) of planning target volume of primary tumor (GTV-T-P)，dose of 95% volume (D95)，dose of 1% volume and 1cc volume of spinal cord and brainstem，mean dose of bilateral parotid gland and the dose volume received more than 30Gy.Statistical analysis was performed by using SPSS 16.0.Results The mean translational error and rotation error along X，Y and Z axis were (1.02±0.83) mm，(1.28±1.08) mm and (1.27±1.06) mm respectively，and (0.79±0.67) degree，(1.00±0.82) degree and (0.73±0.68) degree respectively. The dose differences among brainstem，spinal cord and parotid gland were statistically significant. The difference of GTV-T-P (Dm) was not statistically significant，while D95 was significant different but the average rate of change was 2.18% only.Conclusion Setup error of IMRT affect dose distribution of both target and normal tissue，and the effect is greater on normal tissue than that on target tissue.

Key words: Cone beam CT, Head and neck cancer, Setup error, Dose