Of all metastatic brain cancers, those from lung cancer are the most common. At least 40% of lung cancers will develop brain metastases at some point throughout the course of the disease. Here we focus on the symptoms, treatment and prognosis of brain metastases from lung cancer. I. Definition of brain metastasis from lung cancer Brain metastasis from lung cancer is different from brain tumor, which is a tumor that occurs in brain tissue. Most of the tumors in the brain do not come from the brain tissue itself. Cancer cells are shed from lung cancer tissues, enter the brain through the blood stream or lymphatic system and start to grow. Small cell lung cancer develops brain metastases early, often before the tumor is detected in the lung. Once small cell lung cancer develops brain metastases, it is included in the extensive stage. Non-small cell lung cancer also often spreads to the brain, mostly in the late stage of the disease, and once it spreads to the brain, it is classified as stage IV. Symptoms of lung cancer brain metastasis Once lung cancer spreads to the brain, it will destroy the brain tissue, cause inflammation and swelling, thus compressing the brain tissue and causing the corresponding symptoms. The symptoms vary according to the location of metastasis, the number of metastases and general condition. About one-third of patients with brain metastases are asymptomatic when they are detected. When the following symptoms appear, it often indicates that brain metastasis of lung cancer has occurred: 1. headache 2. convulsion 3. loss of balance 4. nausea and vomiting 5. difficulty in walking 6. uncoordinated movements 7. abnormal speech constructions 8. visual changes, such as loss of vision or double vision 9. weakness of one side of the body 10. memory loss 3. Diagnosis of brain metastasis of lung cancer MRI has obvious advantages over CT in diagnosing brain lesions. However, when there are pacemakers, heart stents or other metal objects affected by magnetic fields in the body, only CT of the brain can be done. PET-CT helps in diagnosis when it is not easy to distinguish cancer from scar tissue. When it is clinically difficult to distinguish between metastatic cancer or primary brain tumor, cranial biopsy is necessary to clarify the diagnosis.