Purpose:To systematically evaluate accumulative imaging doses to organs-at-risk and associated lifetime attributable risk(LAR)of cancer incidence caused by various radiological imaging procedures performed in image-guided radiotherapy(IGRT)of cancers.Methods and Materials:With IRB approval,the gender,age,size of 4832 cancer patients treated with IGRT with the various imaging procedures applied,including computed tomography,kilo-voltage portal imaging,megavoltage portal imaging and cone-beam computed tomography,were collected.Correlations between patients size and imaging dose to organs were established via Monte Carlo dose calculations in patient anatomy,and then used for organ-specific dose estimation.BEIR Ⅶ models were applied for LAR estimation considering ones gender,age and accumulative imaging dose.Results:The maximum average LAR of cancer incidence for male brain,female brain,male lungs and female lungs,female leukemia was at age group of 0-19,peaking at 541,657,1330,3380 and 1210,respectively.However,it peaked for male leukemia at age group of 60-79.The LAR of cancer incidence in 98.3%of brains and 99.1%of lungs is lower than 400 and 2000 per 105 persons,respectively.The quantitative correlation between LAR(per 105 persons)of leukemia incidence and accumulative imaging dose(cGy)to red bone marrow(RBM)was fitted with an exponential function.Conclusions:Regardless of sex,accumulative imaging dose caused much higher LAR of cancer incidence in children than in adults in IGRT,by an average factor of 56.8(1.3-520.4)among age groups.However,the dose may increase in prostate cancer patients with frequent application of radiological imaging procedures.The LAR of cancer incidence for most brains and lungs is lower than 400 and 2000 per 105 persons,respectively.The LAR of leukemia incidence showed exponential increase with accumulative imaging dose to RBM.