The lung cancer that occurs in the trachea, main bronchus and segmental bronchus above the third level is the central lung cancer (see the figure below), which accounts for about 3/4 of the lung cancer, and squamous epithelial cell carcinoma, adenoid cystic carcinoma and small cell undifferentiated carcinoma are more common. Figure left Central type lung cancer (lung enhanced CT film), occupying lesions (shown in circles) can be seen in the middle part of the trachea, and the lumen is blocked about 3/4 of the time; Figure right Normal chest CT (control) in the middle of the trachea, the lumen is open Cancer occurring in the bronchus (i.e., within the lung) below grade 4 (below the segment) is called peripheral type lung cancer (see the figure below), accounting for about 1/4 of the time, with adenocarcinoma being more common. Figure left: Peripheral type lung cancer (chest CT film), an irregular mass of 3cm×5cm in the right upper lung, potato-shaped, with an increase in the volume of the right lung; Figure right: CT of normal lung (control), the texture of both lungs is clearly visible, and no mass is seen. Lung cancer can be simply classified into two basic types: small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC). 1. small cell lung cancer (SCLC): it is the most malignant type of lung cancer, accounting for about 1/5 of primary lung cancer. patients are younger, mostly around 40-50 years old, and mostly have a history of smoking. It mostly occurs in the large bronchus near the lung gate. The cancer cells are fast growing, invasive and metastasize early. 60%-100% of blood vessels are found to be invaded during surgery and 80%-100% of lymph nodes are proved to have metastasis at autopsy, often metastasizing to brain, liver, bone, adrenal gland and other organs. This type is more sensitive to radiotherapy and chemotherapy. 2.Non-small cell lung cancer (NSCLC): This distinction is quite important because the treatment options for these two types of lung cancer are very different.NSCLC includes squamous carcinoma, adenocarcinoma, large cell carcinoma and mixed cell carcinoma, etc. ① Squamous epithelial cell carcinoma (referred to as squamous carcinoma) is the most common type, accounting for 40%-50% of primary lung cancers, mostly seen in older men, and is very closely related to smoking. Central type lung cancer is the most common type, and has the tendency to grow into the lumen, often causing bronchial stenosis at an early stage, leading to atelectasis, or obstructive pneumonia. Squamous carcinoma grows slowly and metastasizes late, so it has more chances of surgical resection and 5-year survival rate, but radiotherapy and chemotherapy are not as sensitive as small cell undifferentiated carcinoma. Occasionally, a mixture of squamous carcinoma and adenocarcinoma is seen, which is called mixed lung cancer (squamous adenocarcinoma). Large cell undifferentiated carcinoma (large cell carcinoma): can occur near the hilum or in the bronchi at the edge of the lung. Large cell carcinoma metastasizes later than small cell undifferentiated carcinoma and has a better chance of surgical resection. (iii) Adenocarcinoma: It accounts for about 25% of primary lung cancer. It is more common in women and is not related to smoking. It mostly grows in the mucus glands of small bronchi at the edge of the lung, therefore, adenocarcinoma is the most common among peripheral type lung cancer. It is easy to metastasize to liver, brain and bone, and more likely to involve pleura and cause pleural effusion. Mixed cell carcinoma: The common type is squamous carcinoma combined with adenocarcinoma, or small cell lung cancer combined with squamous carcinoma.