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Background and Purpose: This study presents Danish consensus guidelines for delineation of the heart and cardiac substructures across relevant Danish Multidisciplinary Cancer Groups., Material and Methods: Consensus guidelines for the heart and cardiac substructures were reached among 15 observers representing the radiotherapy (RT) committees of four Danish Multidisciplinary Cancer Groups. The guidelines were validated on CT scans of 12 patients, each with five independent contour sets. The Sørensen-Dice similarity coefficient (DSC), the distance between the centers of the arteries and the mean surface distance were used to evaluate the inter-observer variation., Results: National guidelines for contouring the heart and cardiac substructures were achieved. The median DSC was 0.78-0.96 for the heart and the four cardiac chambers. For the four substructures of the left ventricle, the median DSC was 0.35-0.57. The coronary arteries were contoured in ten segments, with the best agreement for the left anterior descending coronary artery segments, with a median distance between the arteries ranging from 2.4-4.4 mm. The median variation was 3.7-12.8 mm for the right coronary artery segments and 3.7-6.2 mm for the left circumflex coronary artery segments, with the most pronounced inter-observer variation in the distal segment for all three coronary arteries., Conclusion: National guidelines for contouring the heart and cardiac substructures were developed across relevant Danish Multidisciplinary Cancer Groups, where RT dose to the heart is of concern. The inter-observer contour overlap was best for the heart and chambers and decreased for smaller structures., (Copyright © 2020 Elsevier B.V. All rights reserved.)