Purpose:Respiratory gated radiation therapy(RGRT)gives accurate results when a patients breathing is stable and regular.Thus,the patient should be fully aware during respiratory pattern training before undergoing the RGRT treatment.In order to bypass the process of respiratory pattern training,we propose a tumor location prediction system for RGRT that uses only natural respiratory volume,and confirm its application.Methods:In order to verify the proposed tumor location prediction system,an in-house phantom set was used.This set involves a chest phantom including target,external markers,and motion generator.Natural respiratory volume signals were generated using the random function in MATLAB code.In the chest phantom,the target takes a linear motion based on the respiratory signal.After a four-dimensional computed tomography(4DCT)scan of the in-house phantom,the motion trajectory was derived as a linear equation.The accuracy of the linear equation was compared with that of the motion algorithm used by the operating motion generator.In addition,we attempted tumor location prediction using random respiratory volume values.Results:The correspondence rate of the linear equation derived from the 4DCT images with the motion algorithm of the motion generator was 99.41%.In addition,the average error rate of tumor location prediction was 1.23%for 26 cases.Conclusion:We confirmed the applicability of our proposed tumor location prediction system for RGRT using natural respiratory volume.If additional clinical studies can be conducted,a more accurate prediction system can be realized without requiring respiratory pattern training.