Abstract: Objective To estimate the appropriate margins for the planning target volume(PTV)in non-small cell lung cancer 3DCRT.Methods Eight non-small cell lung cancer patients under-went 3DCRTor IMRT.kV-CBCT scan was acquired on couch before and after radiotherapy.The analysis of these images was performed by automatic and manual regist ration of the kV-CBCT and planning CT images.The patient setup was then corrected by moving the couch accordingly.CBCT online-guided correction data were recorded.Results A total of 160 CBCT sets of 8 non-small cell lung cancer patients were analyzed.If the online image guidance was not corrected,the distance between the clinical target volume and the planning target volume was 10.9mm;If every online image guided radiotherapy was corrected,the distance between them was 2.2mm.Conclusion There is some extent of setup error in non-small cell lung cancer 3DCRT or IMRT.The approach based on the CBCT measurements can be used to reduce the impact of setup error obviously and to estimate the required margins for PTV.