**2. Anatomy**

Lungs are a paired structure that is separated into left and right by the mediastinum, which contains the tracheal, heart, esophagus, and lymph nodes. The left lung is divided into upper and lower lobe by oblique fissure, while the right lung is divided into three lobes (upper, middle, and lower) by oblique and horizontal fissures.

Lung cancers can arise from mucosa of the tracheobronchial tree or the alveolar lining cells of peripheral lung parenchyma. Tumor can spread locally within lung parenchyma or invading surrounding structures including mediastinum, major vessels, or chest wall (**Figure 1**). They can also spread along major airways causing obstruction, distal collapse, or atelectasis (**Figure 2**).

**Figure 1.** Tumor invasion to chest wall.

**Figure 2.** Tumor over left main bronchus causing collapse of left upper lobe (red arrow).

There is rich lymphatic within the respiratory system that accounts for the high rate of nodal metastasis. The lymph node map proposed by the International Association for the Study of Lung Cancer (ISALC) in 2009 divides the lymph nodes into 14 stations and sever zones [5]. It is adopted by the latest seventh edition of AJCC and UICC Manual for N staging, with involve‐ ment of ipsilateral hila node as N1, ipsilateral mediastinal nodes as N2, and contralateral medias‐ tinal or supraclavicular nodes as N3. Lymph nodes drainage depends on the location of tumors, with those in left upper lobe drain predominantly into subaortic node and those in right upper lobe drain predominantly into right upper paratracheal node. Middle and lower lobe tumors drain more commonly into subcarinal and lower paratracheal nodes. However, skip metastasis to mediastinal nodes bypassing hilar nodes occur in around 10–25% tumors [6]. Lymph nodes with short axis diameter ≥10 mm is considered suspicious of nodal metastasis (**Figure 3**).

**Figure 3.** Enlarged mediastinal lymph node over (a) right upper paratracheal node (station 2R) and (b) subaortic node (station 5).
