**6. Target volume delineation**

#### **6.1. Conventional radiotherapy**

Any clinical information and findings from bronchoscopy and mediastinoscopy should be gathered and correlated with diagnostic images. For postoperative cases, surgical records and pathology reports should be reviewed. And in case with doubt, discussions with surgeons and pathologists are encouraged to identify sites at risk of recurrence. Clips that are placed intraoperatively at sites with incomplete resection are useful for target identification.

An appropriate window setting should be used to delineate different targets on planning CT. Extent of primary lung tumor and mediastinal disease is best visualized by lung setting (window width 1600 and window level ‐600) and soft tissue setting (window width 400 and window level 20), respectively (**Figure 10**). Diagnostic imaging (e.g., CT or PET) should be coregistered with the planning of CT for contouring. PET images can help delineate area of collapse and atelectasis from tumors, but care should be taken when matching the images due to poor spatial resolution and breathing motion artifact.

**Figure 10.** Soft tissue window setting (a) in planning CT to define the mediastinal lymph node (red arrow), which cannot be easily seen in lung window (b).

Gross tumor volume of primary tumor (GTV‐P) is best contoured on lung window setting to include any visible tumor within lung parenchyma and the speculated border. Any local inva‐ sion to surrounding structures (e.g., chest wall, vertebra) should be included as well based on soft tissue window setting. Areas of collapse or atelectasis were excluded but can be difficult to differentiate. Input from radiologists and PET may be useful. Elective nodal irradiation is not recommended as isolated nodal recurrences are rare. So GTV of lymph node (GTV‐N) will only include any pathologically confirmed lymph nodes (fine needle cytology or core biopsy) and any suspicious lymph nodes based on imaging characteristics (including short axis diameter ≥1 cm, necrotic center or PET uptake). If chemotherapy had been used before radiotherapy, all initial sites of tumor involvement should be contoured unless it exceeds a tolerable radiotherapy portal.

An isotropic margin is then added to GTV‐P to cover microscopic tumor spread to form the clinical target volume (CTV‐P). Usually, a 6 mm margin for adenocarcinoma and an 8 mm margin for squamous cell carcinoma are used as it had been shown to cover around 95% of microscopic tumor extension on pathological specimens [11]. Subsequent CTV‐P is edited according to the presence of natural barriers (e.g., great vessels, bone). For GTV‐N, usually no additional margin is needed for CTV.

A margin from CTV to PTV (planning target volume) depends on tumor motion and daily set‐ up errors (**Figure 11**). Tumor motion can be quite variable from zero in cases using implanted fiducial markers in image‐guided radiotherapy to certain centimeters in cases without any breathing motion control. Set‐up errors are regularly measured in each department and usually within 5 mm in all directions. In common practice with free breathing treatment, a 1 cm isotro‐ pic margin is usually given to form the PTV. But a larger superior‐inferior margin of 1.5 cm may be used for tumors with greater movement as long as the lung dose is within the tolerance limit.

**Figure 11.** Target volume delineation: primary lung tumor (T) is contoured on lung window as gross tumor volume (GTV; red line); an additional 6 mm is added to form clinical target volume (CTV; blue line) to cover microscopic spread; further 1 cm margin is added to form the planning target volume (PTV; green line) to account for tumor motion and set‐up error.

For palliative radiotherapy using AP beams, information about tumor extent from diagnostic imaging can be superimposed on those visible on simulator to form the GTV. And a further 1.5 cm margin from GTV can be used to define the radiotherapy field border.

#### **6.2. Stereotactic body radiotherapy**

**6. Target volume delineation**

Any clinical information and findings from bronchoscopy and mediastinoscopy should be gathered and correlated with diagnostic images. For postoperative cases, surgical records and pathology reports should be reviewed. And in case with doubt, discussions with surgeons and pathologists are encouraged to identify sites at risk of recurrence. Clips that are placed

An appropriate window setting should be used to delineate different targets on planning CT. Extent of primary lung tumor and mediastinal disease is best visualized by lung setting (window width 1600 and window level ‐600) and soft tissue setting (window width 400 and window level 20), respectively (**Figure 10**). Diagnostic imaging (e.g., CT or PET) should be coregistered with the planning of CT for contouring. PET images can help delineate area of collapse and atelectasis from tumors, but care should be taken when matching the images due

Gross tumor volume of primary tumor (GTV‐P) is best contoured on lung window setting to include any visible tumor within lung parenchyma and the speculated border. Any local inva‐ sion to surrounding structures (e.g., chest wall, vertebra) should be included as well based on soft tissue window setting. Areas of collapse or atelectasis were excluded but can be difficult to differentiate. Input from radiologists and PET may be useful. Elective nodal irradiation is not recommended as isolated nodal recurrences are rare. So GTV of lymph node (GTV‐N) will only include any pathologically confirmed lymph nodes (fine needle cytology or core biopsy) and any suspicious lymph nodes based on imaging characteristics (including short axis diameter ≥1 cm, necrotic center or PET uptake). If chemotherapy had been used before radiotherapy, all initial sites of tumor involvement should be contoured unless it exceeds a tolerable radiotherapy portal. An isotropic margin is then added to GTV‐P to cover microscopic tumor spread to form the clinical target volume (CTV‐P). Usually, a 6 mm margin for adenocarcinoma and an 8 mm margin for squamous cell carcinoma are used as it had been shown to cover around 95% of microscopic tumor extension on pathological specimens [11]. Subsequent CTV‐P is edited

**Figure 10.** Soft tissue window setting (a) in planning CT to define the mediastinal lymph node (red arrow), which cannot

intraoperatively at sites with incomplete resection are useful for target identification.

to poor spatial resolution and breathing motion artifact.

**6.1. Conventional radiotherapy**

36 Radiotherapy

be easily seen in lung window (b).

Internal target volume (ITV) takes into account both GTV and internal tumor motion. It can be generated from the 4D CT using the maximum intensity projection (MIP) scan, maximum inspiratory, and expiratory scans, or all 10 phases of respiratory cycles (**Figure 12**). No CTV is needed. The usual CTV to PTV margin is 3–5mm, but it depends on methods of immobiliza‐ tion, tumor motion assessments, and treatment verification.

**Figure 12.** Target delineation on 4D CT: tumor is contoured on MIP images to form the internal target volume (ITV; red line); addition 5 mm margin was used to generate the planning target volume (PTV; green line).
