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Objective To evaluate CBCT image quality and its long-term stability for HUs in different scanning conditions.Materials and Methods With the OBI of Varian iX system,Catphan 504 phantom in the different modes :SDH(standard dose head)、LDH(low dose head)、HQH(high quality head)、PSL(pelvis spot light)、Pelvis 、LDT(low dose throx)was scanned through a year.7 datas of CTP515,CTP528 and CTP404 modules in TomoCon PACS 3(TatraMed,Bratislava,Slovak Republic)software system were collected.It read HU values in the same area of different density sticks(Air 、PMP 、LDTE 、Polystyrene 、Acrylic 、Delrin 、Teflon)in CTP404 with 7 different scanning conditions,and HU-ED curves with OriginPro 8.0 were created.Analysis of variance(ANOVA)was used to evaluate the results for the different scanning techniques.Results The results of low contrast resolution under different scanning conditions met the requirements of acceptance test procedure.In high contrast resolution,LDT(5Lp/cm)and Pelvis(6Lp/cm)were inferior to the other 4 scanning conditions(8Lp/cm).The accuracies of spatial linear distance and angle were winthin ±1%.HUs in Air density plug were constant(s:0.46-7.95, M=1.62),but HUs of other density plugs with different conditions have the significant difference(F=660.84,p=0).Under LDH,the difference of HUs(40.92 ± 5.05)was maximum.The standard deviations of CT numbers in different density plugs were decreased with the increase of scanning voltages and currents.Under different conditions in a year,the mean HUs show a good agreement(SDH:F=0.205,p=0.664; LDH:F=0.101,p=0.760;HQH:F=0.098,p=0.763;PSL:F=3.532,p=0.102;Pelvis:F=0.199,p=0.669 ; LDT:F=0.013,p=0.912).Conclusions HUs of CBCT image under different scanning conditions exist a little differences,but there have a good consistency for the long term stability analysis.The little difference of creating the HU-ED curves is helpful for CBCT images directly used for dose recalculation in adaptive radiation therapy.