Figure 1.

Sources and formation processes of brain metastases. (A) The most common sources of brain metastases are lung cancer (40%-50%), followed by breast cancer (15%-20%), skin cancer (mainly melanoma) accounting for 5%-10%, and gastrointestinal malignancy (4%-6%). (B) Tumor cells and secreted vesicle contents can disrupt the integrity of BBB, thereby promoting tumor metastasis to intracranial, interacting with surrounding astrocytes, microglia/macrophages, and then influencing the biological behavior of brain metastasis through various pathways such as secreting cytokine networks, direct contact and exosomes, and establishing complex networks. (C) Tumor cells can produce mutual adaptive changes with the components of the central nervous system microenvironment, and it is this interaction that determines the occurrence and development of brain metastatic lesions. (D) Metastatic foci appear in the skull, producing obvious mass and edematic effects, which seriously affects the quality of life of patients.