It is very common to see metastasis during chemotherapy. On one hand, there is a certain percentage of efficiency of chemotherapy drugs and individual differences in sensitivity, and on the other hand, it is very common to see drug resistance during chemotherapy drug treatment. The primary focus appears to shrink, which proves that the tumor is still sensitive to chemotherapeutic drugs. The reason why brain metastases appear is that microscopic metastases in the brain may already exist before chemotherapy, and the vast majority of chemotherapeutic drugs cannot cross the blood-brain barrier (BBB), so the intracranial metastases cannot be controlled. Prophylactic irradiation of the brain is controversial because the cumulative dose of radiotherapy to the brain is certain, and if the metastases are found after prophylactic irradiation in advance, the metastases can no longer be treated with the full dose of radiotherapy.