1.Classification by anatomical site 1.Central type lung cancer: cancer occurring above the segmental bronchi to the main bronchi, accounting for about 3/4, with squamous epithelial cell carcinoma and small cell undifferentiated carcinoma being more common. 2. Peripheral type lung cancer: cancer occurring below the segmental bronchus, accounting for about 1/4, with adenocarcinoma being more common. 1.Squamous epithelial cell carcinoma (squamous carcinoma for short): the most common type, accounting for about 40-50% of primary lung cancer, mostly seen in elderly men, and very closely related to smoking. It is most common in central type lung cancer 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, metastasizes late, has relatively more chances of surgical resection and higher survival rate, but is less sensitive to radiotherapy and chemotherapy than small cell undifferentiated carcinoma. 2.Small cell undifferentiated carcinoma (referred to as small cell carcinoma): it is the most malignant type of lung cancer, accounting for 10%-15% of primary lung cancer. Patients are younger in age, hiding around 40-50 years old, and mostly have a history of smoking. It mostly occurs in the large bronchi near the hilum, tends to grow in the submucosa, often invades the extra-tubular lung parenchyma, and easily fuses with the hilar mediastinal lymph nodes to form a mass. The cancer cells grow fast and are highly invasive. 60%-100% of the blood vessels are invaded and 80%-100% of the lymph nodes are metastasized, often to the brain, liver, bone, adrenal glands and other organs. This type is more sensitive to radiotherapy and chemotherapy. 3.Large cell undifferentiated carcinoma (large cell carcinoma): it can occur in the bronchi near the hilum or at the edge of the lung. Large cell carcinoma metastasizes later than small cell carcinoma and has a better chance of surgical resection. 4.Adenocarcinoma: It is more common in women, and there is an increasing trend in men. It has little relationship with smoking and mostly grows in the mucus glands of small bronchi at the edge of the lung. Therefore, adenocarcinoma is the most common among peripheral lung cancers. Adenocarcinoma accounts for about 25% of primary lung cancer and is more likely to metastasize to liver, brain and bone than squamous carcinoma. It is more likely to metastasize to liver, brain and bone, and more likely to involve the pleura and cause pleural effusion.