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Radiotherapy (RT) is widely used in the adjuvant and curative treatment of thoracic cancers, such as mediastinal lymphoma, breast, lung and esophageal cancer due to an improved overall survival. However, thoracic RT may cause incidental RT dose to the heart with a subsequent risk of radiation induced heart disease (RIHD). The aim of this study was to provide and validate national guidelines for delineation of the heart and substructures across relevant Danish multidisciplinary cancer groups (Danish Breast Cancer Group, Danish Lung Cancer Group, Danish Lymphoma Group and Danish Esophageal, Gastroesophageal and Ventricle Cancer Group) where RT dose to the heart may be of concern.

Preliminary cardiac contouring guidelines were developed and tested among 13 oncologists and two radiation physicists representing the RT committees of four Danish multidisciplinary cancer groups. Consensus guidelines for the heart and cardiac substructures were subsequently agreed upon by all 15 observers. For validation, the guideline was used for delineation by each observer on 4-6 different non contrast CT scans resulting in 12 patients each with 5 independent contour sets totaling 1500 independent contours (fig 1). The DICE similarity coefficient (DSC) and the distance between the centers of the arteries were used to evaluate the inter-observer variation after guideline consensus.  

National guidelines for contouring of the heart and cardiac substructures was achieved. The inter-observer contour overlap was best for the heart with a median DSC at 0.95. For the four cardiac chambers, the median DSC was 0.81-0.90 with the best accordance for the left ventricle (fig 2). The inter-observer variation was high for the four sub-segments of the left ventricle with a median DSC of 0.49-0.68. The coronary arteries were contoured in nine sub-segments, with the lowest variation for the left anterior descending coronary artery with a median distance ranging from 2.4-4.4mm. The variation was 3.7-12.8mm for the right coronary artery and 3.7-6.2mm for the left circumflex coronary artery with the most pronounced inter-observer variation in the distal sub-segment for all coronary arteries.  

National guideline for contouring the heart and cardiac substructures have been developed across relevant Danish multidisciplinary cancer groups where RT dose to the heart are of concern. The inter-observer contour overlap was best for the heart and chambers and decreased for smaller structures. The guideline and validating contours will be used to develop an automated segmentation atlas.