The classification of lung cancer is not very uniform yet, and there are various classifications according to the site of tumor occurrence, visual pattern, histopathology and clinical characteristics. 1. Central type Tumor occurs in the bronchi above the segment, i.e. in the lobar bronchi and segmental bronchi. 2.Peripheral tumor occurs in the bronchus below the segment. 3.Diffuse type Tumor occurs in fine bronchi or alveoli, diffusely distributed in both lungs. The tumor is confined to the larger bronchial cavity, with polyp-like or cauliflower-like protrusion into the cavity, and a few of them have tips. The tumor may spread along the wall of the tube in the shape of a tube sleeve, and most of them have no infiltration outside the tube wall. The tumor invades the larger bronchial tube wall, the mucosal folds of the tube wall disappear, and the surface is granular or granulation-like. The wall is thickened, the lumen is narrowed, and often infiltrates into the lung tissue outside the wall. The bronchial wall structure is still present in the cross-section of the mass. 3.Nodular type The mass is round or round-like, less than 5 cm in diameter, and when it is clearly demarcated from the surrounding tissue, the edge of the mass is often small lobulated. 4.Mass-shaped masses are irregular in shape, larger than 5 cm in diameter, with large lobulated edges and poorly demarcated from the surrounding lung tissue. 5.Diffuse infiltrative type The tumor does not form a limited mass, but is diffusely infiltrative, involving most of the lung lobes or lung segments, similar to lobar pneumonia. 1. squamous cell carcinoma, including spindle cell (squamous) carcinoma; 2. adenocarcinoma, including adenoid ductal adenocarcinoma, papillary adenocarcinoma, fine bronchial lung cancer, alveolar cell carcinoma. 3.Adenosquamous carcinoma. 4.Undifferentiated carcinoma is divided into small cell carcinoma (including oat cell type, intermediate cell type, compound oat cell type) and large cell carcinoma (including giant cell carcinoma, clear cell carcinoma). 5.Carcinoid tumor (endocrine tumor of lung). 6.Bronchial adenocarcinoma includes adenoid cystic carcinoma, mucinous epidermis-like carcinoma, and adenoid alveolar cell carcinoma. The biological behavior of small cell lung cancer is significantly different from other epithelial carcinomas (squamous carcinoma, adenocarcinoma, adenosquamous carcinoma, large cell carcinoma), i.e., it is clinically highly malignant, has extensive distant metastasis at an early stage, and is more sensitive to chemotherapy and radiotherapy, thus the treatment principles are different from other epithelial carcinomas. Therefore, from a clinical point of view, there is a worldwide tendency to coarsely classify these two types of lung cancers with different biological behaviors into small cell lung cancer (SCLC) and non-small cell lung cancer (NSCLC), which includes epithelial cancers other than small cell carcinoma.