Symptoms of brain metastases are mainly reflected in both neurological symptoms and physical signs, and are generally caused by the transfer of tumor cells from the primary and other parts of the body into the skull. Tumor metastasis consists of a series of complex biological events, roughly through the following processes: 1) gene activation, amplification, deletion, or inactivation of inhibitory genes; 2) neovascularization; 3) malignant proliferation of cells; 4) evasion of host immune attack; 5) tolerance to drug therapy; 6) invasion of the tumor by expression and activation of metastasis-related genes; 7) clonogenic growth of the tumor cells by secretion of growth and angiogenic factors at the site of metastasis through adhesion molecules, changes in the activity of proteolytic enzymes, and cellular movement. 7. Tumor cells grow clonally through adhesion molecules, changes in protease activity and cellular movement, and secretion of growth and angiogenic factors at the metastatic site. Pathology of brain metastases 1. Primary tumors at home and abroad believe that lung cancer is the most common type of brain metastases, accounting for about 50%. Gastrointestinal and breast cancers are the second most common, genitourinary and skin cancers are less common, and sarcomas and germ cell tumors are the most common among children. However, clinically, there are quite a number of patients who cannot find the primary lesion, and even after surgery for brain metastases, the source of the tumor still cannot be determined. Lung cancer, breast cancer and melanoma are prone to early metastasis, on the contrary, urinary tract tumors; colon cancer, breast cancer and renal cancer often have single metastasis, lung cancer, melanoma and tumors with unknown site are prone to multiple metastasis; pelvic tumors are prone to subcerebellar single metastasis, accounting for 50%, and the reason is unknown. Metastatic pathway (1) via lung – blood circulation – brain: it is the most common pathway, most of the primary tumor cells go through this pathway to intracranial brain tissue. Lung cancer, breast cancer, skin cancer, etc. mainly metastasize through this pathway (2) Direct invasion: Tumors in neighboring areas, such as nasopharyngeal carcinoma, retinoblastoma, olfactory neuroblastoma, malignant tumors of the scalp and skull can directly invade the cranium and metastasize into the brain (3) Through the subarachnoid space: Very few tumors of spinal cord metastasize to the cranium through this pathway, such as gliomas or ventricular meningiomas. Orbital tumors invade into the skull along the optic nerve sheath and spread in the subarachnoid space. (4) Translymphatic system: Tumor cells enter the cerebrospinal fluid circulation into the skull along the spinal nerves or lymphatic space around the cranial nerves or invade into the skull through the vertebral veins.