Lung cancer is divided into two types, small cell lung cancer and non-small cell lung cancer, according to its cell morphological characteristics and biological behavior. Small cell undifferentiated carcinoma The cell morphology of small cell undifferentiated carcinoma is similar to oat grain, so it is also called oat cell carcinoma. It accounts for about 20% of all lung cancers. Small cell undifferentiated carcinoma mostly originates from the larger bronchi, so most of them are central lung cancer. Small cell undifferentiated carcinoma has low differentiation and fast growth, so lymph node metastasis and hematogenous metastasis can occur at an early stage, and lymph node metastasis is extensive, and the volume of metastatic lymph nodes is often larger than the primary lung lesion. Brain metastasis and bone metastasis are the most common distant metastases. Small cell undifferentiated carcinoma has the worst prognosis among all lung cancer types. It is highly sensitive to radiation and chemotherapy, but the majority of tumors will recur after treatment is stopped. The treatment plan for small cell undifferentiated carcinoma is obviously different from other types of lung cancer, emphasizing systemic chemotherapy as the main treatment, supplemented by local radiotherapy or surgery. Non-small cell lung cancer Other than small cell undifferentiated carcinoma, other types of lung cancer are collectively called non-small cell lung cancer, and there are mainly the following types: 1. squamous epithelial cell carcinoma. Squamous carcinoma is more closely related to smoking and generally originates from larger bronchi, which means it is mostly central lung cancer. Squamous carcinoma generally grows more slowly, has a longer course, and develops distant metastasis later, and usually first metastasizes to lymph nodes via lymphatic circulation before bloodstream metastasis occurs. Among all types of lung cancer, the highest percentage of lung cancer is treated by surgical resection. Adenocarcinoma mostly originates from the smaller bronchial mucosa epithelium, so it is mostly located in the peripheral part of the lung (i.e. peripheral type lung cancer) and is spherical in shape, near the pleura. Adenocarcinoma is more common in women and is not significantly associated with smoking. Adenocarcinoma of the lung often has no obvious symptoms in the early stages of the disease and is often detected during chest x-ray or chest CT examination. Some cases of lung adenocarcinoma are diagnosed only after systematic examination when symptoms of distant metastasis appear. The incidence of lung adenocarcinoma has gradually increased in recent years and has occupied the first place of all lung cancer types in western countries, which may be related to the aggravation of passive smoking and atmospheric pollution. 3.Alveolar bronchial carcinoma is a special type of adenocarcinoma of the lung. The cancer cells grow and spread along the alveolar wall and bronchial wall without destroying the lung tissue structure. Most bronchoalveolar carcinomas are located in the peripheral part of the lung field and appear as isolated or multiple round or oval nodules with infiltrative growth, which may involve the whole lung or both lung lobes and often involve the pleura and produce pleural effusion. 4.Undifferentiated large cell carcinoma Undifferentiated large cell carcinoma is a rare type of lung cancer, more than half of them originate from larger bronchus, with high malignancy and huge size, and lymph node or bloodstream metastasis can occur without obvious symptoms in early stage. 5.Bronchial adenoma Bronchial adenoma is a primary low-grade malignant tumor originating from mucus glands or adenoidal epithelium of bronchial mucosa. Its incidence is low, accounting for only about 2% of all lung cancer types. It mostly occurs in larger bronchi, grows slowly, has clear borders with rich blood flow, often erodes neighboring tissues, and can also occur distant metastasis, and is prone to local recurrence if not completely excised. Bronchial adenoma is mostly found in young women, and the common clinical symptoms are irritating cough and coughing up blood. Bronchial adenoma is divided into the following types: (1) Bronchial carcinoid tumor The most common type of bronchial adenoma, most of them originate from silver-loving cells containing neuroendocrine granules in the larger bronchial mucosa. It grows mainly under the bronchial mucosa and may burst into the bronchial lumen to form a polypoid mass with smooth surface and rich blood vessels, which is prone to bleeding. Some bronchial carcinoid tumors grow both inside and outside the bronchial cavity, forming masses both inside and outside the bronchus. Bronchial carcinoid tumor usually grows slowly and has a long history of disease. The treatment effect is very good after surgical resection, and the survival rate can be more than 80% in 5 years after surgery. However, lymph node metastasis and distant metastasis can occur in some atypical carcinoid tumors, and the prognosis is poor. (2) Bronchial adenoid cystic carcinoma, also known as cylindrical adenoma or cylindrical tumor, is relatively rare. Most of them originate from bronchial mucosal ducts or secretory glands, and most of them occur in the lower end of trachea or proximal main bronchus. Adenoid cystic carcinoma is more malignant and often involves the bronchial wall and its surrounding lung tissues, leading to bronchial obstruction, as well as lymph node metastasis and distant metastasis. (3) Mucinous epidermoid carcinoma is the least common type of bronchial adenoma, which originates from the mucus glands in the bronchial mucosa and is often polypoid and tipped. Mucinous epidermoid carcinoma is less malignant and has good surgical results. In addition, in a few patients, two or more different histological types of components can be present in the lung tumor at the same time. For example, there is a squamous component in adenocarcinoma tissue or an adenocarcinoma component in squamous tissue, and a non-small cell carcinoma component in small cell undifferentiated carcinoma. This type of lung cancer is known as mixed lung cancer.