
What is a brain tumor? A brain tumor is an abnormal mass of tissue in which the cells grow and multiply without restraint, apparently unregulated by the mechanisms that control normal cells. One factor that distinguishes brain tumors from other tumors is that they arise in the skull, an organ encased by bone, and there is very little room for expansion with the skull. They are also among the few types of tumors that generally do not tend to metastasize or spread to other parts of the body.