**8. Radiotherapy planning**

**7. Organs at risk delineation**

38 Radiotherapy

at risk volume).

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

structure called lung minus PTV after subtraction.

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

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 For radical treatment, three‐field conformal radiotherapy is most commonly used. The choices of beam numbers and beam angles depend on the location of the tumor and proxim‐ ity to OARs. For early stage I–II tumors with lateralized target volume, a lateral, anterior, and posterior oblique beams are usually chosen to reduce irradiate contralateral lung (**Figure 14**).

**Figure 14.** Beam arrangement in a three‐field conformal radiotherapy for early stage lung cancer (red line = GTV; green line = PTV).

For more advanced stage disease with tumor involvement to mediastinum or across midline, the above three‐field technique using ipsilateral beams only may not give good dose cover‐ age to target, and addition of contralateral beam will increase total lung dose. In such case, two phases treatment should be considered. First phase will treat the mediastinum using AP beams shaped by multileaf collimator (MLC), while the second phase will use conformal tech‐ nique to give adequate coverage to all the target volume. With this approach, total lung dose can be reduced but OARs near midline (e.g., esophagus, spinal cord) will receive higher dose.

For palliative radiotherapy, anterior and posterior fields modified by MLC are usually used with dose prescribed to midplane. Energy of photon beam used will depend on separation at the center of the field.

Radiotherapy plans should be carefully evaluated using dose‐volume histogram (DVH). Optimal plan should aim at 95% PTV receiving at least 100% of the prescribed dose and 99% PTV receiving a minimum of 90% of the prescribed dose. For OARs, commonly used dose constrains for lung minus PTV is V20 (volume receiving >20 Gy) below 35%, preferably below 30%. However, a tighter constrain to reduce the risk of radiation pneumonitis should be con‐ sidered when there is presence of other risk factors including preexiting lung disease and concurrent use of chemotherapy. Another frequently used limit is the mean lung dose below 20 Gy. The dose constrains for other OARs are maximum dose to spinal cord less than 45 Gy and heart V20 less than 40 Gy. Care should be given to avoid irradiation of more than 10 cm length of the esophagus due to higher long‐term risk of stricture.

For SBRT, either intensity‐modulated radiotherapy using 6–8 fields (IMRT) or rapidarc ther‐ apy is recommended to deliver a high and conformal dose to a precise area (**Figure 15**). Dose to skin should be minimized to avoid cutaneous and subcutaneous toxicities. Recommendations to other OARs can be made reference to that published by ROSEL study and RTOG 0813 study.

**Figure 15.** Beam arrangement and dose color wash from SBRT for lung cancer using IMRT technique.
