Brain cancer is a serious medical condition characterized by the uncontrolled growth of abnormal cells within the brain or the central spinal canal. These growths are known as tumors, which can be either malignant (cancerous) or benign (non-cancerous). Even benign tumors can be dangerous because the skull is a rigid structure; any growth can increase pressure on critical brain tissue, impairing function. Tumors are classified as either primary or secondary. Primary brain tumors originate within the brain itself. Secondary occur when cancer cells from another part of the body—such as the lung, breast, or skin—spread to the brain.