Brain cancer. Light micrograph of a section through a glioma in the brain. A glioma is a tumour that derives from glial cells in the central nervous system (CNS). They most commonly occur in the brain where they often infiltrate extensively throughout the normal brain structures. Here the tumour has completely replaced the normal brain tissue. Brain gliomas can cause headaches, nausea and vomiting, seizures, and cranial nerve disorders as a result of increased intracranial pressure.