Primary cancers of many tissues and organs throughout the body can easily metastasize to the lungs to form metastatic lung cancer, such as choriocarcinoma, cervical cancer, ovarian cancer, breast cancer, testicular cancer in the reproductive system; stomach cancer, colon cancer, liver cancer in the digestive system; nasopharyngeal cancer, lung cancer in the respiratory system; kidney cancer, bladder cancer in the urinary system; thyroid cancer, adrenal cancer in the endocrine system; osteogenic sarcoma, fibrosarcoma, malignant Melanoma, etc. Most metastatic lung cancers are caused by the metastasis of the primary cancer. Bloodstream metastasis of cancer is most likely to occur in tissues and organs with abundant capillaries and vascular networks, and the lung is one of such organs with an especially large vascular network. In terms of physiological anatomy, the lung is the “first filter” of the large circulation, and the pulmonary circulation has a dual vascular distribution of pulmonary arteries and bronchial arteries, so the lung is a good site for blood-borne metastasis of many cancers. The time of lung metastasis of cancer can occur first or later, early or late. In some cases, metastatic lung cancer is found at the same time as the primary cancer; in some cases, metastatic lung cancer is found first and the primary cancer is found only during further investigation; in more cases, metastatic lung cancer is found after the primary cancer has undergone radical treatment such as surgery and radiotherapy for some time. Another distinctive feature of metastatic lung cancer is that its clinical manifestations are less obvious, and its clinical symptoms are lighter and fewer compared with those of the primary lung cancer. About 2/3 of patients have no symptoms, and another 1/3 of patients only have mild cough, shortness of breath, chest pain and sputum blood, and some of them may have pestle and mortar finger and joint pain. It is this lighter and less symptomatic feature that causes many patients to be unaware of the occurrence of metastatic lung cancer. Therefore, all cancer patients should be followed up and reviewed regularly after treatment. Because metastatic lung cancer is mostly blood-borne metastasis with multifocal characteristics, i.e., the number of lesions can be large and the scope can be wide, its diagnosis depends on imaging examinations such as X-ray, CT or MRI (magnetic resonance imaging), etc. X-ray films can reveal scattered multiple nodules in unilateral or bilateral lung fields and masses in the hilum and mediastinum, and can also identify the presence of pleural effusion; CT and MRI can show the location, number and volume of metastatic cancer foci, and can be associated with inflammation, tuberculosis and other diseases. It can also be differentiated from benign lesions such as inflammation and tuberculosis.