Abstract: BACKGROUND AND AIM: To evaluate DNA damage induced by radiotherapy in nasopharyngeal cancer (NPC) patients. MATERIALS AND METHODS: Comet assay was employed to detect comet frequency , tail length and Olive moment of peripheral blood lymphocytes from 19 NPC patients undergoing radiotherapy at the accumulated dose of 0,2,4,8,10 and 16 Gy. RESULTS: After radiotherapy , the comet frequency and tail length in peripheral lymphocytes rose significantly at the dose of 2 Gy(P<0.01 and P<0.05) whilst Olive moment rose significantly at the dose of 8 Gy(p<0.01). The comet frequency , tail length and Olive moment all increased following radiation dose in this study(0～16 Gy) ,with a clear dose-response relationship. CONCLUSION: Radiotherapy caused DNA damage in NPC patients at low dose, demonstrating dose-effect relationship between radiation dose and comet frequency , tail length and Olive moment.

Key words: radiotherapy, nasopharyngeal cancer, comet assay