The two main cell types of lung cancer include non-small cell and small cell lung cancer, with non-small cell lung cancer being the most common. As the name implies, non-small cell lung cancer cells appear large and irregularly shaped under the microscope. Non-small cell lung cancer accounts for 85% of all lung cancers. Even within non-small cell lung cancer, there are many cell subtypes, the most common of which are three: adenocarcinoma, squamous carcinoma, and large cell carcinoma. At this point, it is mainly up to the pathologist to analyze and type the tumor tissue microscopically, and if necessary, special staining is needed to identify them. Adenocarcinoma Adenocarcinoma is defined as a tumor cell that originates from glandular epithelial cells or has glandular secretion. In lung adenocarcinoma specifically, the tumor cells are mucus-secreting epithelial cells that originate from the bronchi. Adenocarcinomas are usually located at the peripheral margins of the lungs, or near the tiny bronchioles. In the United States, adenocarcinoma accounts for 40% of all primary lung malignancies. Adenocarcinoma is most common in both men and women, but is more prominent in women. Adenocarcinoma is also most common in nonsmoking populations. In Asia, women account for 70% of non-smoking lung cancers. In the last century, China was dominated by squamous lung cancer, but in recent years adenocarcinoma has significantly surpassed squamous cancer as the most common pathological type in China, and the current distribution of pathological types is essentially the same as in the United States. Since most adenocarcinomas are located in the periphery of the lung, they are far from the large bronchi and do not produce obvious respiratory symptoms due to tumor invasion and obstruction, so they are relatively insidious. However, adenocarcinoma can easily spread to the pleural cavity, first involving the dirty pleura (outer surface of the lung), and then possibly metastasizing to the pleural cavity, resulting in malignant pleural effusion, but it usually appears only in advanced stages. Many adenocarcinomas, especially small ones (<2 cm< span="">) are detected during physical examination or examination for other diseases. The clinical presentation and course of adenocarcinoma is highly variable. Some adenocarcinomas will progress rapidly, while others show a very slow growth trend, especially in non-smoking women with adenocarcinoma, which grows particularly slowly and responds better to treatment, especially to targeted therapy, often leading to long-term survival. Squamous cell carcinoma arises from thin, flattened squamous epithelial cells in the respiratory tract that look somewhat like fish scales on the microscope. Squamous cancers account for 30% of all primary malignancies in the United States. Because squamous carcinoma is mostly located within the airway, it is difficult to detect in the early stage. Even after CT examination, it is difficult to detect superficial early lesions due to the interference of bronchial structures, and it is only when the tumor produces bleeding or airway obstruction that it is easily noticed. In recent years, squamous carcinoma has declined significantly in both the United States and China, largely due to the results of smoking cessation. In addition, studies have shown that the advent of filters and low-tar cigarettes has resulted in finer smoke particles that can be easily inhaled deeper into the lungs, thus avoiding damage to the bronchial epithelium. However, this has had the side effect of increasing the incidence of adenocarcinoma in the periphery. Large cell lung cancer Large cell lung cancer is a subtype of non-small cell lung cancer. The two terms sound somewhat similar, but they are two different cell types and are related in a hierarchical way. Large cell lung cancer looks like cancer cells are larger and round under the microscope. Most large cell lung cancers are located in the peripheral areas of the lungs. Sometimes large cell lung cancer is also called undifferentiated carcinoma and is the least common type of non-small cell lung cancer. Sometimes several other cell types are seen mixed within the tumor tissue of large cell lung cancer, making it somewhat difficult to diagnose.