Brain metastases are common intracranial tumors and are the leading cause of death in cancer patients. Take the common lung cancer brain metastases as an example. The mechanism of intracranial metastasis of lung cancer is not fully understood yet. The route of intracranial metastasis is mainly through hematogenous metastasis. Lung cancer grows fast and tumor is prone to necrosis and shedding due to insufficient blood supply. Lung tissue is rich in blood vessels, and cancer cells enter the body circulation through the pulmonary veins and travel up the carotid artery or vertebrobasilar artery to brain tissue to form metastases. Brain metastases can be single, but more patients tend to have multiple metastases, appearing in any part of the skull and brain in large or small size, growing rapidly, often accompanied by cerebral edema, necrosis, and even bleeding, and patients have headache, dizziness, nausea, vomiting and other symptoms. If the tumor grows in a sensitive part of the brain, the patient may experience seizures, numbness and weakness of the limbs, and aphasia. Most of the patients with brain metastasis had known cancer tumor in other parts of the body before and then developed brain symptoms; however, there are also some patients who had not known cancer tumor in the body before and showed brain symptoms first, and then were diagnosed with brain metastasis after examination. Most of the patients with brain metastases are not in the late stage of cancer, so if they are diagnosed early and treated with appropriate methods, they can obtain satisfactory results, which not only prolongs the survival period of patients, but also improves their quality of life. The average survival period of brain metastases without treatment is 1 month; the average survival period of external irradiation treatment alone is 3 months. The average survival of patients with brain metastases treated with Gamma Knife radiosurgery at the Second Artillery General Hospital in Beijing is significantly prolonged. Gamma knife not only can treat single brain metastases, but also has superiority in the treatment of multiple brain metastases, because it does not need to operate, and patients in poor physical condition can also receive treatment. Gamma knife treatment cycle is short (1 to 3 days), does not affect the treatment of the primary tumor, and the treatment is not limited by the location and number of tumors, with its practical, safe, effective and simple, it has become the preferred treatment method for more and more patients with brain metastases.