It has been thought that the blood-brain barrier (BBB) has a tight connection between endothelial cells and an additional layer of glial cells than the general capillary wall, which has no cellular drinking effect and is lipophilic, and only small molecules with molecular weight less than 5000D can pass through it, while most chemotherapeutic drugs have a large molecular weight, and most anti-tumor cytotoxic drugs are difficult to pass through the BBB. Recent studies have found that in the process of brain In the process of metastatic carcinoma, the complete blood-brain barrier cannot be formed locally in the brain; when the metastatic carcinoma in the brain is 0.2mm, the BBB can be destroyed by some factors, so that it loses its normal anatomical and physiological characteristics and allows the chemotherapeutic drugs to enter the brain tissue, and its brain response rate can reach 50%. It has also been further demonstrated that radiotherapy has the effect of opening the BBB for a window of 1-3 months. This new concept described above further advances the development of chemotherapeutic agents for the treatment of brain metastases.