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Objective: Radiation-related heart disease,appearing as a late toxicity following thoracic radiotherapy (RT),is increasingly becoming a concern and should not be disregarded,especially in patients with curable malignancies such as breast cancer and Hodgkin lymphoma.There may be a heterogeneous biological response to radiation among cardiac substructures such as the myocardium,coronary arteries,pericardium,and cardiac conduction systems.The aim of this study was to quantify the displacement of cardiac substructures,including the anterior myocardial territory (AMT),left ventricle,and coronary arteries,during a normal cardiac cycle.Materials and methods: Computed tomography (CT) images with retrospective electrocardiographic gating of 17 eligible patients were obtained.All images were reconstructed automatically for the end-diastolic and end-systolic phases.CT scanning without contrast at a random phase and a selected vertebral body were used as references to measure 3-dimensional displacements of the cardiac substructures.The spine was used as a stable reference to measure the displacement between the end-diastolic and end-systolic phases (Dd-s),between the random and end-systolic phases (Dr-s) as well as between the random and end-diastolic phases (Dr-d).Results: The displacement between the end-diastolic and end-systolic phases (Dd-s) was greater than that between the end-systolic and random phases and between the end-diastolic and random cardiac phases.The largest displacements for the heart were in the left,posterior,and inferior directions with an average Dd-s of approximately 4–6 mm.The average Dd-s for the AMT and left ventricle was 1.2–2.7 mm in the anterior and right directions,4.3–7.8 mm in left and posterior directions,and 4.9–6.3 mm in superior and inferior directions.For the coronary arteries,the average Dd-s was 2.8–5.9 mm in the anterior-posterior direction,3.5–6.6 mm in left-right direction,and 3.8–5.3 mm in the superior-inferior direction.Inter-observer agreement was excellent for the heart,AMT,and left ventricle (kappa coefficient,>0.75 for all) and good for most coronary arteries in 3 dimensions (kappa coefficient,0.511–0.687).The Dd-s did not differ significantly between men and women.Conclusions: For most of the 3-dimensional directions measured in this study,average displacements of cardiac substructures,including the heart,AMT,left ventricle,and the coronary arteries,ranged from 3 to 8 mm between the end-diastolic and end-systolic phases.The greatest cardiac motion was observed in the left,posterior,and inferior directions.These findings will be useful for the accurate estimation of the radiation dose to cardiac substructures and a better understanding of the risk of radiation-related heart disease.