ObjectiveTo analyze the transitional shifts between with different sets of cone-beam computed tomography (CBCT) and the planning CT for liver cancer patients,and calculate the margins from clinical target volume (CTV) to the planning target volume (PTV)with and without image guided radiotherapy (IGRT).MethodsFive liver cancer patients received radiotherapy after transcatheter arterial chemoembolization (TACE).The first CBCT images (CBCT1) were obtained with Elekta CBCT plus active breathing control (ABC) system before treatment.The second CBCT images (CBCT2 ) were obtained after correcting the set-up errors and the third CBCT images ( CBCT3 ) were obtained after treatment.The CBCT images were registered and matched with the planning CT images using lipiodol as a direct surrogate for target localization.The PTV margins were calculated by comparing the shift between planning CT and CBCT according to formula M =2.5 ( Σ doctor2 + Σ set-up2 + Σ transter2 )1/2.Paired t-test was used to compare the differences between the results from CBCT1,CBCT2 and CBCT3. Results The average transition shifts in the left-right ( LR),superior-inferior (SI) and anterior-posterior (AP) directions were 0.254,-0.612,0.314 cm between planning CT and CBCT1 ;were 0.020,0.014,-0.064 cm between planning CT and CBCT2 ;and they were -0.004,0.042,-0.040 cm between planning CT and CBCT3.The PTV margins were LR 0.96 cm,SI 0.96 cm and AP 0.83 cm without IGRT,and LR 0.67 cm,SI 0.68 cm and AP 0.58 cm with IGRT.ConclusionsThe PTV margins can be reduced by 3 mm with IGRT for liver cancer using lipiodol as a direct surrogate for target localization.