Lung cancer originates from the bronchial mucosal epithelium, but can also originate from the bronchial glands or alveolar epithelium. Carcinomas originating from the mucosal epithelium usually grow into the bronchial lumen or penetrate the adjacent tube wall to infiltrate the surrounding lung tissue. Cancer cells may spread through lymphatic, hematologic or transbronchial metastasis. The malignancy, growth rate and metastasis of cancer are related to the histological type, differentiation degree and other biological characteristics of lung cancer. Lung cancer is located in the right lung more than the left lung, and in the upper lobe more than the lower lobe. Lung cancer originating from the main bronchus and lobar bronchus is located close to the hilum, which is called central type lung cancer. In 1998, the International Association for the Study of Lung Cancer and the World Health Organization classified lung cancer into 9 types according to cell type: 1. squamous cell carcinoma; 2. small cell carcinoma; 3. adenocarcinoma; 4. large cell carcinoma; 5. adenosquamous carcinoma; 6. polypoid, sarcoma-like or sarcoma-containing carcinoma; 7. carcinoid tumor; 8. salivary gland carcinoma; 9. unclassified carcinoma. According to statistics, 40%-50% of cases have a single histological type, and the rest are a mixture of different types of carcinomas. The following four types of lung cancer are commonly seen clinically: 1. Squamous cell carcinoma (squamous carcinoma): it accounts for about 40%-50% of lung cancer. Patients are mostly over 50 years old, with men accounting for the majority, and are closely related to smoking. It is generally believed that squamous carcinoma comes from squamous metaplasia or atypical proliferation of basal cells and develops through the stages of atypical proliferation, carcinoma in situ and microinfiltrating carcinoma. However, it can also develop directly from normal bronchial mucosa. Squamous carcinomas often occur in the lobar and main bronchi and tend to develop into polyps or non-tipped masses that obstruct the lumen and cause obstructive pneumonia. The degree of differentiation of squamous carcinoma varies, but generally the growth rate is slow and the course of the disease is long. Squamous carcinoma is more sensitive to radiation and chemotherapy and has relatively good prognosis. 2.Small cell carcinoma: The incidence rate is lower than that of squamous carcinoma, accounting for about 15%-20% of primary lung cancer, which is a kind of lung cancer with low differentiation and high malignancy. It usually originates from larger bronchi and is mostly central type lung cancer, infiltrating the bronchial wall and causing luminal narrowing. The cells are equivalent to 2-4 times the size of lymphocytes, with little cytoplasm, resembling a naked nucleus, with chromatin-filled nuclei and nucleoli of similar size, shaped like oat grains, hence the name oat cell carcinoma. Small cell carcinoma is highly malignant, fast-growing, and has early lymphatic and extensive hematologic metastasis. Although it is more sensitive to radiation and chemotherapy, the prognosis is poor. 3.Adenocarcinoma: The incidence rate has been increasing in recent years and is close to 30% of squamous cell carcinoma. The age of onset is younger and women are relatively more common. Most of them originate from small bronchial epithelium and often appear as peripheral type lung masses, which are often detected during chest X-ray. The growth is generally slow, but sometimes bloodstream metastases occur early and lymphatic metastases occur later. It responds poorly to radiation and chemotherapy and has a poor prognosis.