There is no consensus on the histological classification of lung cancer. Lung cancer is usually classified into the following main types according to the morphological characteristics of cancer cells: 1. Squamous epithelial cell carcinoma is referred to as squamous carcinoma. Among the various types of lung cancer, squamous carcinoma is the most common, accounting for about 50%. Squamous carcinoma mostly originates from the larger bronchi and is often the central type of lung cancer. The degree of differentiation of squamous carcinoma varies, but the growth is generally slow. The course of squamous carcinoma is long and metastasis occurs late, and usually metastasis occurs first through lymphatic tract and only in advanced stage through bloodstream. The surgical resection rate is higher, and it is less sensitive to radiation and chemotherapy than undifferentiated carcinoma. 2. Undifferentiated small cell carcinoma is also called oat cell carcinoma because its cell morphology resembles oat grains. Undifferentiated small cell carcinoma accounts for about 20% of all types of lung cancer. The age of onset is young, mostly seen in men, and most patients have a history of smoking. It usually originates in the larger bronchi and is mostly a central lung cancer. Small cell carcinoma has the worst prognosis among all types of lung cancer because of its low differentiation, rapid growth, early lymphatic metastasis and extensive metastasis to distant organs and tissues via blood vessels. Small cell carcinoma is highly sensitive to radiation therapy and/or anti-cancer drug treatment. Adenocarcinoma mostly originates from small mucus-secreting epithelial cells of the bronchial mucosa. It is more common in female patients, and the age of onset is also younger. It accounts for about 20% of all types of lung cancer. Adenocarcinoma is not closely related to smoking, but in some cases it occurs on the basis of pulmonary fibrotic scar lesions. Adenocarcinoma usually has no obvious clinical symptoms in the early stage and is often detected during chest X-ray examination. The cancer grows slowly, but in some cases, bloodstream metastasis occurs early, and more often, the primary cancer in the lung is found only after presenting brain metastasis symptoms. 4.Alveolar carcinoma of fine bronchus it is a special type of adenocarcinoma with low incidence. It accounts for about 3% of all types of lung cancer and is more common in women. The cancer is often located in the peripheral part of the lung field, with good differentiation and slow growth. This type of lung cancer may have a close relationship with scarring lesions caused by inflammation in the lung. Alveolar carcinoma of the fine bronchus mostly appears as isolated or multiple round nodules, often involving the pleura, and in a few cases as a diffuse infiltrate throughout a lung segment, lobe or both lungs, resembling pneumonia or cornified tuberculosis. The cancer cells grow along the bronchoalveolar ducts and alveolar walls and often secrete mucus. Bronchoalveolar carcinoma rarely metastasizes through lymphatic or blood channels, but it often involves the pleura and produces pleural fluid or spreads widely through the airways, leading to respiratory failure. 5.Undifferentiated large cell carcinoma is rare, about half of them originate from larger bronchus, with large size and high malignancy, and metastasis through lymphatic or blood channels occurs early, sometimes it is found only after brain metastasis is detected, and the prognosis is poor. 6.Bronchial adenoma is a group of primary lung and bronchial tumors originating from submucous glands and epithelial cells of bronchial tubes. The incidence is low, accounting for only about 2%. This group of tumors is slow-growing, with clear borders to the naked eye, but often encroaches on adjacent tissues, and distant metastases can occur. It is easy to recur locally without complete excision, so it should be classified as low grade malignant tumor. Bronchial adenoma often occurs in larger bronchial tubes, which are rich in blood vessels and have a young age of onset, mostly in women. The common clinical symptoms are cough, hemoptysis and obstructive emphysema, atelectasis or lung infection caused by the tumor blocking the bronchial lumen. There are several types of bronchial adenomas: (1) Bronchial carcinoid tumors are the most common among bronchial adenomas. Ninety percent of them occur in large bronchial tubes and are central tumors, while 10% occur in small bronchial tubes and are peripheral tumors. Carcinoid tumors mainly grow under the bronchial mucosa and protrude into the bronchial lumen to form polyp-like masses with smooth surface and rich blood vessels, which can bleed easily. In some cases, the tumor grows both inside and outside the bronchial wall, forming a mass in the bronchial lumen and in the lung, which can produce clinical symptoms such as cough, hemoptysis, bronchial obstruction and paraneoplastic syndrome. Generally, bronchial carcinoid tumor grows slowly and the course of disease can be as long as 5-8 years, but some cases, especially a few atypical carcinoid tumors, can metastasize to local lymph nodes or distant metastases through blood channels. The effect of surgical treatment of bronchial carcinoid tumor is good, and the survival rate of 5 years after surgery can be more than 80%. (2) Bronchial cystic adenoid carcinoma, also called cylindrical adenoma, is relatively rare among bronchial adenomas. It originates from the adenohypophysis or bronchial mucosal secretory glands. Most of them occur in the lower trachea or proximal main bronchus. They are more malignant and often erode the tracheal or bronchial wall and surrounding tissues, causing obstruction of the bronchial lumen and lymph node or distant metastasis. (3) Mucinous epithelioid carcinoma is the least common among all types of bronchial adenoma. It originates from the mucosal secretory glands of the bronchial lobe of the lung and is often polypoid with intact mucosa and mucus secretion. In addition, a few lung cancers can have different histologic types in different parts of the same tumor. More commonly, adenocarcinoma may have squamous carcinoma within adenocarcinoma, or squamous carcinoma may coexist with undifferentiated small cell carcinoma. This type of lung cancer is called mixed lung cancer. In a much smaller number of cases, two or more primary lung cancer lesions may be present simultaneously or sequentially in the same lung or in both lungs, called multiple primary lung cancers. These lesions may be of different histological types, or they may be of the same histological type but have different sites of origin, and the lesions may be independent of each other.