**7. Organs at risk delineation**

Organs at risk including heart, esophagus, and spinal cord will be contoured using soft tis‐ sue window. The heart includes the whole structure within pericardial sac starting from the pulmonary artery to the apex. All layers of esophagus will be included and contoured from the cricoid cartilage to the esophagogastric junction. Spinal cord will be contoured at least 10 cm above and below PTV. For tumors over upper chest, the ipsilateral brachial plexus should also be contoured. Both left and right lungs are also contoured and then used to form a new structure called lung minus PTV after subtraction.

For SBRT, the tracheal and proximal bronchial tree should be contoured as well. Trachea will start from the level of cricoid cartilage to 2 cm above the carina, where it then con‐ tinuous with the proximal bronchial tree (PBT, **Figure 13**) including the distal 2 cm trachea, main carina, bilateral main bronchi, bilateral upper lobe bronchi, lingular bron‐ chus, intermedia bronchus, right middle lobe bronchus, and bilateral lower lobe bronchi. A 2 cm margin applied around the PTB will then be used to form a PRV (planning organ at risk volume).

**Figure 13.** Definition of proximal bronchial tree.
