Primary brain tumors originate in the brain itself. Primary brain tumors do not spread from the brain to other parts of the body except in rare cases. Pathologists classify primary brain tumors into two groups: glial tumors (gliomas) and nonglial tumors. Gliomas are composed of glial cells, which include astrocytes, oligodendrocytes, ependymal cells, Schwann cells, microglia, and satellite cells. Nonglial tumors develop on or in structures within the brain, such as nerves, blood vessels, and glands.
Metastatic or secondary brain tumors begin as cancer in another part of the body. Some of the cancer cells may be carried to the brain by the blood or may spread from adjacent tissue. The site where the cancerous cells originated is referred to as the primary cancer. Metastatic brain tumors are often referred to as brain metastases or lesions. Metastatic brain tumors are the most common brain tumors. Because people are surviving primary cancers for longer periods of time, there has been an increase in metastatic lesions.