Lung 5 cm mass density shadow with clear boundary is generally not serious, mostly benign lesions, pneumonia, lung abscess, lung malformation tumor and other diseases can lead to. 1. Pneumonia: Pneumonia is mostly caused by streptococcus, staphylococcus, viruses and other pathogens, and the severity should be judged according to the area of infection and clinical symptoms. When accompanied only by fever, cough, sputum and other symptoms, usually can be cured. When the respiratory rate is too fast or there is obvious consciousness disorder, it is more serious, and severe pneumonia has a high mortality rate. 2. Lung abscess: Lung abscess is a purulent inflammation of lung tissue caused by bacteria, viruses and other microorganisms, and may present symptoms such as high fever, cough, coughing and spitting up foul-smelling sputum. It can be cured through active examination and treatment. 3. Lung malformation tumor: it is a benign tumor formed by the combination of various normal tissues in the bronchial wall, with complete envelope, slow growth, clear boundary, and there can be calcification points, which is typically manifested as popcorn-like calcification. Appearance of lung 5 cm mass density shadow border clear, although mostly benign lesions, but pneumonia, lung abscess treatment is not timely, there will be serious lethality, it is recommended to timely admission to the hospital to prevent delayed treatment.