A brain tumor is an out of control growth of abnormal cells in the brain. Primary brain tumors start in the brain, whereas secondary brain tumors form in a different part of the body and metastasize (spread) to the brain. This document focuses mainly on primary brain tumors. A primary brain tumor is either benign or malignant. Malignant tumors are cancerous, meaning they can invade nearby tissue and spread throughout the brain. Benign tumors are not cancerous, but may still cause serious problems. Benign tumors may grow larger and spread through the spinal fluid, but rarely infiltrate the brain. Growth of malignant or benign tumors can put significant pressure on your brain due to the fixed size of your skull. For the same reason, malignant brain tumors can spread throughout the brain, but rarely ever spread to other parts of the body.