(This is a dynamic list and may never be able to satisfy particular standards for completeness. You can help by expanding it with reliably sourced entries.) A brain tumor is an abnormal growth of cells within the brain or inside the skull, and can be cancerous (malignant) or non-cancerous (benign). Just over half of all primary brain tumors are malignant; the rest are benign, though they may still be life-threatening. In the US in 2000, survivors of benign primary brain tumors outnumbered those who had cancerous primary brain tumors by approximately 4:1. Metastatic brain cancer is over six times more common than primary brain cancer, as it occurs in about 10–30% of all people with cancer.